I know this sounds silly.. try removing the CD rom and any floppy devices and try agian.
Your PSU may be to blam. And BTW< the vid card is awful. That thing has crappy little render lines everywhere no matter what resolution or refresh.
Ok, here's the deal. I decided to build a cheap, but quick system recently, and I just got it together a couple days ago... It has a L4VXA2 motherboard, with a 20GB HD (temporary), 512MB of RAM, a NVIDIA RIVA TNT2 16MB 2/3D XP AGP VIDEO card, a 2.5GHz Celeron processor, a NEC DVD-RW drive, and a 300W Rosewill power supply.
I've tried to install Windows XP, and when the setup gets to the point of "starting windows", the computer will restart. Out of nowhere, it just shutsdown and starts back up. I tried installing Windows 98 also, but it does the same thing!!! I didn't spend a LOT of money on this system, but I spent enough to where I would LOVE to be able to use it. How do I stop this???