Is it possible to use Windows?
If so, you might be in "safe-mode". If you wonder what Microsoft says about this mode, they say;
Safe mode is a troubleshooting option for Windows that starts your computer in a limited state. Only the basic files and drivers
necessary to run Windows are started. The words "Safe Mode" appear in the corners of the display to identify which Windows mode you are using. If an existing problem does not reappear when you start in safe mode, you can eliminate the default settings and basic device drivers as possible causes.
Now, if you can go into Windows 7, if it's ugly or not, try to right-click on your desktop with your cursor, and go to Personalize, see if it's has changed the theme.
Maybe Windows has been chosen the graphics, animations, transparentness etc. due to your computers performance!
Eventho, your problem seems more difficult, we'll figure it out together, don't worry. You came to the right place!
Did you have experienced any errors and so on? Have you been installing any software that could cause your problem?
Let's get some other suggestions on from other members, otherwise my suggestion would be to do a System Restore. But I would wait to hear some other suggestions first. System Restore
If you cannot get into Windows, i'm sure you have a DVD you can bootup from, and do your restore, but again, let other people come with their suggestions.
Hope this helps!