The issue with a lot of these MAC themes is that they physically change a lot of system resource files. The good ones (bricopak is one for XP), backup all the original files it changes and thus allows you to uninstall the changes. A lot of these themes are not that well written and do not backup anything because of this the only real way to remove is to re-install
System restore will sometimes work but as some of the files normally changed may not be included in the restore this can be a bit hit and miss.
One thing to try is to check on the website of the theme pack and see if they have any removal instructions
Alternatively you could Google for removal instructions for the specific pack you have used
If it's possible to borrow a Vista DVD a repair install may cure the problems but it may be that a complete re-install is the only solution
Repair Install For Vista