Try disabling McAfee and see what happens.
I have a story about MCAfee for you. A roughly $20,000 4 business day loss to McAfee. The communications software with the gas chromatograph - mass spectrometer system I was using was experiencing random communications errors resulting in an inability to run any samples reliably. (It was Win98 on a Dell Optiplex G1 or something - I still dread the machine), Tech flown in power monitors installed the works. It was McAfee interfering with serial communications!!!! ARG! Un-effing-believable. I have been extremely wary of McAfee ever since.
Give it a whirl, if you're worried, just shutdown, disconnect your internet cable from the computer, then startup, disable McAfee, reboot, and see if the undesirable behaviour persists. It shouldn't take long to test. However, be careful in disabling McAfee, it often says it is disabled but is not. (at least I have observed that behaviour) - temporarily disable any services run in its name in addition to it's internal "disable" features (prior to stopping the services). A little extra info there to help make for a valid test.
Mind you, my Vista install just bluescreened while watching youtube not twenty minutes ago. I'm running Microsoft Security Essentials only for AV type stuff currently.