Reading http://grownupgeek.com/dell-PBR-2-failure sounds hopeful. As machines age we get to open them up to do things like that. I also clean them (read http://forums.cnet.com/5208-19681_102-0.html?messageID=3196452&tag=forums06;posts#3196452 )
At 4.5 years you should be getting close to that fifth cleaning.
As to Mcafee (am I repeating myself?) I would uninstall it and try something lighter weight such as the PC Tools Spyware Doctor that is free with the Google Pack or NOD32.