I've had my desktop PC in for repairs 5 times for dropping into a slow running state. It was HPs top of the line 2 years ago and has plenty of horsepower. It runs fine for a few howers. Then, it starts taking 20 to 30 minutes just to open a folder. Boot up takes 30+ minutes. The Geek Squad seems to think mother board. They've run all the diagnostics and cleaning programs. They eliminated the hard drive, too,
if so they cant be very good if they havent solved the problem yet--hope they didnt charge you every time
i would think the last suspect would be your motherboard
why not try running a linux live cd to see if you get the same slowdown--or a virtual machine--or run it in safe mode
if it doesnt slow down in safe mode its usually a good indicator that its not a hardware fault but a software one--you can also run msconfig and tell it to load a diagnostic startup to try to see if its a software fault
personally i would suspect either a software issue or memory or hard drive in that order--how did they eliminate the hard drive?