kulki: I have almost the same situation with a system I put together for a customer. I started with a barebones from Fry's: Duron 950MHz chip on a SoyoTek mobo. He says it keeps resetting on him, especially when he's online. I originally set up a dual boot, Win98SE and WinXP but subsequently removed XP.
When I built the system, in late October he said it was resetting so I replaced the 512MB stick of SDRAM with a 256MB stick. That seemed to resolve the problem. Out the blue last week he called and said that after about a week it started up with the random resets again. (Why he waited nearly 4 months to contact me, I'll never know.)
Anyway, I got the system from him and I want to settle this once and for all. I did a little research in other threads. The concensus in those was that the culprit could be a faulty power supply, video drivers or the mobo itself.
I'll let you know what sort of progress I make. Please let me know if you come up with anything.