If u use windows xp and it is installed in the C: drive then resizing the drive would kill ur installation as XP will not recognize the drive after resizing. Though u can skip that using some registry hacks.
But if u r not using XP or XP is not on C: drive then u can easily change the drive's space in disk management console. To open it simply go to Control Panel > Administrative Tools > Computer Management and then go to the Disk Management under the storage tab.
I have done it using GParted. A student of mine had a laptop with a single partition (like all these crazy manufacturers supply). I resized 80 GB to 20 GB. When I rebooted it said new hard disk found.......blah blah, your device is installed and ready to use. I rebooted again for safety...you know windows
No problems after that.
i think there are variuos softwares to do this ...
is there a way ... to dod it in vista... want to know .. but since i was using xp long back... i did it with partion magic or manager .. iforgot the name..