strange problems with windows xp

Hi I was hoping someone could give me some insight.suddenly my winsdows updates access has been disabled,I can`t get my system restore to function and my explorer 6 browser is not working propperly,(Many web pages won`t open and I often get prompts indicating that I`m using a pre 4.00 version of explorer. I have the latest updates with norton 2003 antivirus ,and sygate firewall.I have reloaded windows twice and reinstalled explorer 6 twice..I have downloaded the blaster detection files and scanned my computer (says I don`t have it)Also I have repeatedly scanned with norton( no virus found).Is it possible that I have could still have a virus.It would seem that something very strange is happening. Any help would be greatly appreciated. thanks Al