Computer crashing

I keep getting crashes with my computer... Sometimes everything just freezes, the sound, screen, everything. I pull out the plug and restart my computer and this can keep on going for two hours until I finally will be able to do something.

What could this be?

Ps. I've done loads of virus searches with a number different anti-virus programmes but they didn't find anything.

How old is your PC seeing it has Vista? If still under warranty send it back.
Freezing could be faulty ram, PSU and sometimes overheating and HDD problems. First check I'd try is Vista's own ram check in safe mode. http://tinyurl.com/2kdbjm
. If ram ok then check the harddrive (read carefully about possible rare problem doing this); http://www.maximumpcguides.com/how-t...windows-vista/ .
If stable in safemode check device manager for any errors and also events viewer.

Also note that you must have only one anti-virus program installed at any one time otherwise you'll get conflicts and problems. Have as many antispyware progs as you like so long as only one has real time scanning.