If you can see the files, you are accessing the drive. Please be more specific. The video card should not interfere with your system drive. It is possible you installed the card in the wrong slot, or got a card that is incompatible with your hardware. Post your bus specs and the card specs here. IE, you can't install a PCI express x16 card in an x4 or x8 slot. Also, you need to be sure your power supply is adequate. Some of these high-performance video cards suck up a LOT of power. That will put the kibosh on the rest of the system, to the point of burning out critical components. Personally (and from experience), I would bet on the power supply.
After I did a restore point in Safe Mode, I could access the files on "My Computer", the DVD drive and USB drive did not show before that. I'll check my system specs tonight and get back to because I'm still experiencing problems with the Nvidea Driver because even after installing the correct driver I still get error messages of Microsoft UAA high definition audio driver not installed, PCI device might not work properly." I think Windows Xp, Service pack 3 has a problem with the sound drivers. Thinking of re-installing Motherboard Sound drivers. I think the different sound drivers are competing for attention and that causes the computer to freeze and then I can't access any files on My Computer. 400 watt power supply is sufficient for GTX 550 ti, I have a PCI x 16 slot for Graphics card. Thank you for advice, I'll give you an update soon.