A crash that restarts the PC is a pretty serious error (it’s called a STOP error - in older versions of Windows NT you’d get the blue screen of death, with a memory dump.)
It’s very difficult to diagnose since there are soooo many things that can cause this.
Does the crash occur only with .avi files? Does it occur with .mpg or .asf or .wmv files? Does it occur with ALL .avi files? Or only some? If some, can you identify the codec of the .avi files that crash (using gspot or avicodec)?
The screenshot showed xvid.dll was the module that crashed. I’m surprised because xvid is a very stable codec (unlike divx for example.) One thing to try is to get the latest version of xvid here http://www.koepi.org/XviD-1.1.2-01112006.exe
Also, have you been installing any new codec packs, or drivers or filters lately? That could be another culprit. If in doubt get later version of the drivers, and uninstall the codec packs. 
PC comes up error,crashes and restarts the pc, anyone else with this problem ?
Avi’s that use to play , now dont play anymore.
All I have to do is just click on __.avi movie once and pc restarts.
there are no viruses on my pc.
I took screen shot of the error message, heres the link.
http://s1.up-file.com/download3/236881244618_k02pq9yigl7yxo43/errorcodec.JPG