I have the exact same PC. I picked it up on the cheap for a multimedia computer. For shits and giggles I tried 5 boxing with it. It was piss poor to say the least. I heard about the TLB issue I have not been tempted to try the fix as it is not my main gaming machine. I have a spare ATI HD 4670 video card that I put in the machine to see if I could get better performance out of it... no joy. RAM is running at about 50% and CPU is topping out at 100%. Unfortunately the power supply is all but welded in the machine and short of adding a second power supply I can not up the video power anymore (not that it is the problem)

Running Northrend my main screen gets 20fps if I do not move or breathe and 10-15fps if I even flinch.

One of the first things I do with a pre-built PC of which I rarely buy is uninstall everything that is not needed. I even disabled gobs of services.

Watch out for the Realtek drivers in this thing, straight out of the box svhost.exe was using 20-25% of the CPU all the time. That particular svhost was pointing to Plug and Play and DComLaunch. A quick uninstall of the Realtek drivers and a restart fixed that problem. There is some sort of issue with the enchanced sound feature from REaltek that keep the processor going. I did not even have to reinstall another version of the sound drivers , the generic Vista HD drivers worked fine. In your process manager you can go to view and turn on some additional columns which will give you some good information. CPU Time was a handy one. I noticed svhost.exe was active 25% of the time which is what brought it to my attention.

I might try the TLB fix later I will let you know what I find.