Computer locks after 30s.

[dubhghaill]

I was playing 'the movies' game last night and went to get something to drink. My machine had locked up and needed restarting. Upon restarting my machine makes it to the log in page and I log in, than in about 30s. my mouse freezes onscreen and ctrl+alt+del no longer works. Same problem in safe mode. Sometimes it won't even log in. Also, if I wait the 30secs. at the log in screen the computer freezes up. Any help would be great.
I have win XP with the latest service packs, and I have not downloaded anything in over a week, so I doubt it's a virus (and I can't run norton antivirus to see).  Someone recommended I run disk cleanup and defrag but I don't know if I can as it always (in normal and safe modes) locks up aftr 30s. 

[Broni]

Some computer info needed: processor speed, amount of RAM, hard drive size/free space.

[dubhghaill]

processor speed = pentium 4, amount of RAM =512, hard drive size = 256gb, free space = 60gb

[Soviet_Genius]

It might be overheating. Try cleaning it out. DON'T USE A VACUUM!

[Broni]

Turn off computer. Open the case. Protect yourself against static, and give it a good cleanup with compressed air.
While there, re-seat RAM. Do you have one, or more RAM sticks?
Don't close the case, turn computer on, and see if all fans are running.
Is it on-board video, or you have video card? If video card - re-seat.