Building a PC

I recently built a pc that has an amd 550mhz processor, 128mb memory, 15G Hd and all the other basics. Software is Win98SE, Microsoft Works Suite, McAfee Anti-virus. The computer runs fine for about a week than it will start to loop (restart itself over and over), the only error message that come up eventually is explorer has performed an illegal operation and will shut down. The only thing that I can think of is that I didn’t set a switch right on the motherboard, sound right, any help would be appreciated.