[Computer freezes randomly, both on and off the internet]

Lately, computer has started to freeze randomly, both on and off the internet. not sure what is going on. have webroot spyware and trend micro anti-virus, have run scans and no problems found. today, system froze after 5 minutes without being on internet, had to use power button to shut down and restart. have been back on and on internet for almost 2 hours now with no problems (knock on wood). seems to happen more frequently when youngest daughter is on myspace or matmice chatting with friends and having multiple tabs open, could these sites or tabs be causing these problems. any advice is appreciated.

 

Could be dozen of reasons from failing CPU fan to failing SMPS.

Could we have the hardware details please ?

How old is the system ?