If you have XP, before formatting to create a new partition, you can try using something like Disk Director..but to do a partition you need to reinstall windows and create a partition right at the beginning of the install after formating your hdd.
I think it lets you resize even the XP partition without creating trouble (and if it doesn't work f!*k it, you were going to format anyway) but as far as I remember it used to create some trouble with a partition where there was installed Vista..
Anyway, Bobby, having different partitions for OS and data ('My Documents' folder included) is highly recommended..