Your computer perhaps came with MS-Windows installed and perhaps two partitions on the hard drive, one called the C: drive and the other called the D: drive. These are simply areas on the disk that have been allocated to each of these ``drives.'' The partition essentially marks where on the disk the boundaries are between the different drives. The raw bits associated with each drive are then organised in some way to create a filesystem. The C: drive and the D: drive probably have the FAT or the FAT32 filesystem. (Under GNU/Linux these drives are usually identified as /dev/hda1 and /dev/hda2.)
These two partitions (or however many your computer came with--perhaps just one) will probably have been arranged to fill the whole of the hard drive, leaving no room for any other operating system. So, if you wish to keep a dual boot system you need to move these partitions to make room for other partitions (with different organisations of the bits--that is, different types of file systems).
Changing the size of an existing partition can damage the data on that partition although by carefully following certain steps this should not happen. Nonetheless be sure to make a backup of the current partitions if you can. This is important if you have data on the system you do not want to lose.
The first step is to decide how much space you need for each of your partitions. For a 6GB hard drive you might give 3GB to Win32 and 3GB to GNU/Linux. But this depends on how much space you are currently using.
Next, the aim is to move all of the data on the current partitions to the beginning of the partition, then change the location of the end of the partition. The Win32 tool fips is commonly used to perform this rearrangement of partitions.
The fips toolkit consists of three files: two executables (RESTORRB.EXE and FIPS.EXE) and a text file (ERRORS.TXT). These should be copied to a bootable floppy (created under Win32 with the DOS command sys a:).
Now under Win32 run your disk de-fragmenter (the DOS command defrag). When finished reboot your computer, booting from the fips floppy disk. Then simply type a:fips and fips will let you know what to do next.