pc keeps rebooting randomly

[mekap04]

I am having problems with my computer restarting out of nowhere. Sometimes when it restarts, it goes all the way through but sometimes it gets stuck and doesn't restart but the power light is still on. I just recently took it in to get repaired and they put in a new power supply. everything was working until that happened. Afterwards, it started with the random reboots/shutdown. Sometimes it would last for 5 minutes before it reboot and sometimes after about 4 hours, it would reboot so its random. could anyone possibly tell me what could cause this. I sent my computer back to the shop to check and they said it ran for them the whole time it was there. Keep in mind I only brought the box/harddrive part of the computer (the main thing). No other cables or parts.

Other info

i checked for dust and cleaned
i also switched outlets to see if it was the outlets
i ran my antivirus and nothing was there

AMD Athlon(tm) XP 3000+
2.16 ghz, 448 MB of RAM

Memory in use 53%
Total Memory 458096 KB
Available Memory 214116 KB

Mainboard Asustek computer A7N8X-LA
BIOS American megatrends (2/24/04)

Austin Power Supply
Model: DR-83203TX
(upgrade from DR-B300ATX)

I also checked system errors and I get these

Tcpip
Dcom
Service control manager

Just now I got the blue screen and it said something about checking disk space, and I done that. Alsoit said something about may need bios update.

[zep516]

I just recently took it in to get repaired

I would consider checking with these people who repaired it almost sounds like a power supply issue again.