[bhudson]

Close as many programs as you can. Click Start->Run. Type %TEMP%, hit enter. The temp window will open. Select everything in it with CTRL-A, hit DELETE. If it complains stuff being in use and cannot be deleted, simply exclude that from the selection and delete again.

Make sure you do this on the temp dir though - you don't want to delete a crucial dir by mistake.

If you still get the same problem, copy the insallation file somewhere else and run it from there.