You can't change size of windows partition containing c:\windows while windows is running
If you boot up from a gparted boot cd you can do it easily
There are a number of other freeware programs to do this for you
Before starting you should make sure the PC is running well
Run chkdsk on each partition
Then empty the recycle bin to free space
I hope the partition you want to shrink had plenty of free space and is imediately after the c windows partition
Shrinking a partition may take a long time unless the drive is almost empty
Look in disk management to confirm this
Usually c: is windows partition which is followed by d: unless d: is your DVD drive
Using disk cleanup make enough free space by removing old restore points, temporary files emptying the recycle bin
Moving the swap file from c: to another drive letter may also free up space
Useful if you have 8, 12, or 16 GB of ram as swap file is usually the same size as the ram
Regards
Mike Barnes