Last night my computer (windows vista) just randomly restarted and when it started back up, the task bar and start menu looked different. Instead of the black vista look, it now is gray and similar to older versions of windows. Also, it won't let me connect to the internet or even open Firefox and Internet Explorer freezes when I try to open it. Has this happened to anyone? How do I fix it? Do I have a virus?
I should add that I have never been able to do a system restore on this computer. It tells me I have errors on the C drive and when I try to fix them it tells me it can't do it while the drive is in use and to schedule it the next time the computer restarts. But it never works.

Probably you might have downloaded or installed some software that has change your taskbar, Well the best thing you can do is restore your computer doing a System Restore.
You can also try going to Control Panel and click taskbar and start menu and check it there.