The best advice? Do what BSAM suggests, download and run Microsoft Security Essentials. Then reformat your hard drive. I have very little confidence in any "anti-virus" to help you. It could happen, I just am pessimistic.
As far as future prevention goes...your router's hardware based firewall is more capable than any software based solution. Add a sandbox like the one that comes free with Chrome and you're golden. This is my opinion and it's worth what you paid for it. I do not use any anti-virus software...I run XP with it's firewall turned off, automatic updates turned off, and no anti-virus installed.
If you are getting the BSOD (Blue Screen Of Death) note what error code(s) it gives you. In my experience your problems may not always be a virus. Memory can go bad and cause problems like freezing.
