How to increase the size of my disc partition without deleting current one

You have, say, a 500 GB drive.
You make two partitions, one is 200 GB, the other is 300 GB.
Now what you want is two partitions of 300 GB each.

Where do you think you can get that extra 100 GB?????????
Pull a rabbit out of a hat?????

You order a large pizza delivered for three people. Just right. Then another couple of people arrive. Come in! Nice to see you! Hold on while I expand this pizza. Hmmm... maybe if I just cut it differently it will get bigger.

EDIT:
To the idea of making two partitions of 300 GB each from a 500 GB drive: Not possible.
To the idea of putting Mac formatted space into NTFS formatted space: Not possible.
The two types of space you can add to NTFS space are NTFS space and unformatted space. You cannot hide one file system inside another.
Solution: Buy an external drive, backup the Windows side, delete that partition, add the unformatted space to the Mac partition, and use Boot Camp Assistant again.

EDIT again:
I wish these PC geeks would go away. You can use partition software to add free space to a Boot Camp partition. The reason you must use Boot Camp to create the partition is that a Mac cannot boot to a standard MBR partition. Boot Camp makes a hybrid partition that Easeus cannot understand. Give up and do it like I suggest using a backup.

What are the benefits of Hard Disk Partitions?

Disk partitioning is the act of dividing a hard disk drive into multiple logical storage units referred to as partitions, to treat one physical disk drive as if it were multiple disks. Partitions are also termed "slices" for operating systems based on BSD and Solaris. A partition editor software program can be used to create, resize, delete, and manipulate these partitions on the hard disk.

Benefits of multiple partitions
Creating more than one partition has the following advantages:
Separation of the operating system (OS) and program files from user files. This allows image backups (or clones) to be made of only the operating system and installed software.
Having an area for operating system virtual memory swapping/paging.
Keeping frequently used programs and data near each other.
Having cache and log files separate from other files. These can change size dynamically and rapidly, potentially making a file system full.
Use of multi-boot setups, which allow users to have more than one operating system on a single computer. For example, one could install Linux, Mac OS X, Microsoft Windows or others on different partitions of the same hard disk and have a choice of booting into any compatible operating system at power-up.
Protecting or isolating files, to make it easier to recover a corrupted file system or operating system installation. If one partition is corrupted, none of the other file systems are affected, and the drive's data may still be salvageable. Having a separate partition for read-only data also reduces the chances of the file system on this partition becoming corrupted.
Raising overall computer performance on systems where smaller file systems are more efficient. For instance, large hard drives with only one NTFS file system typically have a very large sequentially accessed Master File Table (MFT) and it generally takes more time to read this MFT than the smaller MFTs of smaller partitions.
"Short Stroking", which aims to minimize performance-eating head repositioning delays by reducing the number of tracks used per hard drive.[1] The basic idea is that you make one partition approx. 20-25% of the total size of the drive. This partition is expected to: occupy the outer tracks of the hard drive, and offer more than double the throughput — less than half the access time. If you limit capacity with short stroking, the minimum throughput stays much closer to the maximum.

It will help you more if you own a disk partition management software.

How do I deleted a disk partition that is write protected?

What OS ARE YOU USING??? IF IT'S WINDOWS 95,98, OR MILLENIUM, YOU WILL HAVE TO USE FDISK.EXE at the command prompt. IF IT'S NT4.0,2000, XP, VISTA OR "7", YOU CAN USE DISKPART.EXE FROM THE C:\WINDOWS SYSTEM32\ directory at the command prompt instead, or better yet, right click on the start button itself, choose manage my computer, and use the disk administrator tool under the storage tab to list and change partitions at will. right clicking on any of the drives/partitions in the disk administrator tool will bring up the menu of what you can do to each partition.

Or you can download a disk partition management software: http://www.partition-magic.org/

How do you restrict certain hard disk partitions in windows xp for certain users?

you cannot hide disk partitions only folders. if you want your sacred files inaccessible by others put them in "my documents" folder and activate your guest profile. you need to password protect your profile and logout when you intend not to use your pc for a while. other users will log in to your pc as guests and they cannot delete or see the contents of your documents.

why not just change the permission of the partition drive you can do it. just log as administrator, right click the partition, sharing and security. from there you can click advance and the permission.

Does removing disk partition clean all the data?

Removing the disk partition will take away easy access to the drive, but it's not really destroyed.

The disk partition (in simple terms) organizes the data into a group, which can then be assigned a drive letter, Deleting the partition removes the 'group' assignment, but the data remains largely intact. 

The danger in just removing partitions is that any semi-tech-savvy individual could easily recover the entire contents of the drive, with very basic data recovery software.

If you want to avoid your time-consuming disk cleaning, you could wack the exposed circuit board with a hammer a couple times (the method I use). 

At least with that kind of damage, any would-be 'dumpster diver' would need substantial technical skills (and probably a class 100 cleanroom, or around $2000 cash for a professional recovery from a facility, such as Knoll-Ontrack or DriveSavers) to recover the data.

HOWEVER, if you are responsible for "data security" in your line of work, and your work is sensitive or interesting enough for outside entities to take an 'active interest' in the contents of your trashed drives, then you should probably be physically opening the drives, and smashing the platters..takes about 5 mins per drive. Data recovery from this type of destruction requires an electron microscope, which is exactly what it would take to recover data from a high-end software-based 'drive cleaning'/data shredder' app.