Presario SR5050NX freezes during boot

Yuffie

So I recently installed a new graphics card (GeForce 8600 GT) and a
power supply. Everything was working great. I installed the new drivers
for the graphics card. Then there was no sound. Re-installed the sound
drivers and everything was dandy again. Computer restarted and booted
fine so I turned it off and called it a night. Now when I turn the PC on
it freezes at the start-up screen. The Compaq logo shows, the fans are
working but the screen just hangs. It won't enter BIOS or safe mode.
Tried to put in Vista CD but it won't work. Checked to make sure all the
connections were secure from the the power supply. Reseated the graphics
card. Nothing has worked. Any suggestions?

Compaq Presario SR5050NX
Pentium D 3.0 Ghz
250 GB HDD
Upgraded 2048 MB memory RAM
Running Vista Home (32-bit)