Know this is an old thread, yet may help many others in the future!
I had the same problem.
Installed new Video Card. (EVGA GeForce 8400 GS 1G DDR3)
VERY slow boot up, very sluggish general operation, Video was slow as could be, Computer would eventually hang after 20 minutes or so (Sometimes sooner).
It turned out to be a wrong setting in the Bios.
Had to set the BIOS to use the PCI-e slot for video first.
Once I did that, it runs great!
Fast & smooth!
Hope this helps.