firstly see if it is software or hardware to blame.
you may have a trojan, malware etc. --check your pc with an online scan for viruses at symantec or panda websites.
-check you haven't got sasser or w32.blaster - download "fixblast".
-download and install spybot seek and destroy, ad-aware. run them to clean the hard drive.
you may have to download these at a friends or internet cafe then run them on your pc if you cant access the net or if its slow.
finally, to stop a recurrence,use a good firewall and antivirus (UP TO DATE!)
if it keeps freezing after all that, check/replace the following hardware:
-cooling fans in PC and on processor may be dead or clogged with dust-add more in the tower case if it runs hot - AMD processors tend to.
-Power supply unit - new ones about 20 pounds. Get 300 watt or more.
-Buy a better CPU tower case with more space and airflow