Freeze up

When you say freeze - Is it a complete system freeze where Ctrl-Alt-Del doesn't work or just a application freezez where Ctrl-Alt-Del still operates. If the former I would suggest checking the following: 1) CPU temperature and fan - make sure it's not overheating. 2) OS - Check for latest SP's etc. 3) Newly added hardware/drivers - has anything changed here lately? These may be good things to check first. Certainly the CPU as 20mins could be enough for it to get too hot and to stop in its tracks

Alt+Ctrl+Del does not work and I replaced the cpu fan.

Check other hardware such as power supply,then download free mem test and test your memory
Check the temp to make sure that system is not overheating
You mentioned changing the cpu fan,is it possible the system was overheating before being detected,which will cause the cpu to have problems

is there anything happening after it "freezes", will ctrl+alt+del bring up task manager, does the mouse move?

If you are getting system beeps (from the base unit) it is possible that you have an IO IRQ problem, it is most likely that it is something you do, or something that is inadvertantly initiated after 20minutes-ish.

If you move the mouse, and click alot, (but the screen freezes), or type stuff, and try shortcut commands, and here your defined windows beeps for error, or close or whatever, it would meant that your grafix card has died.......

It is possible that the system is overheating, and not necessarily your CPU.....open your case, and get a desktop fan, and blow it into the case. You would be supprised how many HDDS, RAM Modules and GFX cards overheat.

Hi,

When you replaced the cpu fan did you apply a thermal transfer compound?

You can also try The Ultimate Boot CD available
from

http://www.ultimatebootcd.com/

If you run this you can do a complete diagnostic of your computer and find out what is going wrong and why.

Col