You say in your first post that you only notice a loss of performance and pop ups when surfing the internet. This would indecate that your problem is not hardware related.
This would suggest that your install of McAfee has caused the problem. You say you have tried to reinstall windows but are getting errors, i take it you have tried to reinstall over the original installation and this is where you are getting problems.
Re-boot to your install disk and choose the option to delete your original partition. Then go through the proccess of recreating a new partition and installation.
You can check inside your box to make sure that everything is seeted properly befor you do this just to be sure.