This requires you to use regedit.
NOTE: Before proceeding, you should backup up your registry, if not your computer. For whoever doesn't already know, regedit is a powerful tool that, if not used properly, can make Windows stop working.
With that said, (and my legal disclaimer out of the way), let me proceed.
You will find the key to edit at HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Setup\SourcePath . Change this value to whatever directory you want. Be sure to include the "\" at the end of the key.
Jeremy Hannon
Brainbench "Most Valuable Professional" for Computer Technicians