How to change the default installation drive/folder/directory in Vista

Steps to change the ProgramFilesDir registry value to use the default location for the Program Files folder:

1. Click Start, click Run, type regedit , and then click OK.

2. Locate and then click the following registry subkey:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion

3. In the details pane, right-click the ProgramFilesDir registry value, and then click Modify.

4. In the Value data box, type the default location for the Program Files folder, and then click OK.NoteThe default location of the Program Files folder is systemdrive\Program Files. For example, if Windows is installed on drive C, type C:\Program Files in the Value data box.

5. Exit Registry Editor.

Ref:

http://support.microsoft.com/default.aspx/kb/933700