Freezing can be the result of overheating - are all your fans working properly?
Can you start it in safe mode? If so, I'd also do scans with antispyware and anti-Trojan software as well as the antivirus: I'd scan with A Squared, Windows Defender, Spybot and Adaware.
Also, how many antivirus packages do you have running? You only need one: more than one will cause problems.