Oh, yeah... To answer your cry for help:
PLEASE LIST THE MODEL TYPE OF YOUR HP PAVILLION
I'm assuming that the 300GIG HDD you are talking about is external. That being assumed, you would need firewire support to get anykind of performance out of that drive. You didn't have any firewire ports on your motherboard or other devices so you needed to install a Firewire PCI card. With the PCI card installed your PC would shutdown automatically. (I believe that first, this could be a IRQ conflict with that PCI slot that you installed the firewire card into. There are several ways to fix this problem. Mostly this can be fixed by movig the PCI card to a different PCI slot. The other ways involve some adjustments in the BIOS.
THe second problem you are facing, seems to be that you didn't properly uninstall the other Firewire card drivers before installing the second firewire card and drivers. If this was done properly, when you installed the second card, rebooted, windows would recongnize that you have installed a new card and then it would have asked you to install the drivers and the new software. At which time / If you installed the new card & drivers properly, you would have gotten the same reboot/freeze/lockup problem you had with the first one. Due to the IRQ conflict.
The last cause could be the PSU, but i doubt it. I'm sticking with the IRQ conflict here.