cant play games without crashes

whenever i play a game exactly after 18 minutes it quits the game.

this has happened in all of my games but if it helps here are the titles.

dawn of war: dark crusade

half life 2: deathmatch 

What are you system specs?

Also, it could be overheating or having the wrong drivers.

What anti-virus software are you running?...might be a virus...might not

without a little more info it is hard to say... 

ummm If it is happening exactly every 18 minutes like clockwork then that would probably rule out things like overheating, driver issues, operating system problems or any other system spec problems, seeing that most of which usualy result in the following problems:

1. prevent the game from working at all

2. Result it a game crash sooner then 18 minutes

3. Or at very least Result in random crashes 

I have heard of Viruses that cause system reboots after a a certain time but i havn't heard of one that causes game crashes. (which i think was due to a virus activating the system restart timer)

 is the problem only with games or does it happen when running other programs (eg. Windows media player or msn Explorer?)