I need to talk to my mate Daz about this one - but you haven't recently installed SP2 have you? That puts a Microsoft sized sticking plaster over the chasms of ooze that they left open originally.
If so and you are running a seperate anti V/Internet security package, the two could potentially clash. If so, go to control panel/security centre and turn off the MS bells and whistles. Nearly all third party firewall/antiV packages should recognise this, but there is a chance that a few oddballs might not.
Here to help if I can and blue flashing light fitted to wig...
Conan