uxtheme.dll

or whatever they call it in the Control Panel.
It would be under the Programs something-or-other. (Whatever it is they now call Add/Remove Programs.) Sorry, I can't remember the Vista names for these things.

Once you are in there, add support for themes.
If the box is already checked, uncheck it, go through the procedure, then go back and check it, and finish the procedure again.

Have you scanned for viruses/malware?

You might need to go into your BIOS (press keyboard key: F1, F2 or DEL), go to the "boot" section and make sure that your cd/dvd drive is made to boot first. then click on "exit and save". Now your computer will boot from your cd/dvd drive, when you see the "install" icon on the Vista window, look down to the bottom left of your screen, you will see "repair" option, click on this to do a full repair. This should sort out your problem.
If not you can download uxtheme.dll here:
http://tinyurl.com/7t6dc

Update:
This little program is only for:
Windows 95/98/Me/NT/2000 or Windows XP. Are you SURE you want to install this on Vista?

Please post back if you have any more problems or questions.