As long as it is not the drive holding the system, you can change it using
Disk Management. Click start/run, type diskmgmt.msc and click ok. Right
click a drive to begin. Again, you cannot under any circumstances change the
drive letter assigned the system partition. The only way to do that is to
start all over again.
Control Panel - Admin Tools - Computer Management, select Disk
Management and look lower right for the graphic of the drive. R-click
in a partition (other than the system's one) or on a CD icon, Change