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.