There are a couple of possible problems and solutions here. A main one is the power supply. Adding a high-capacity hard drive can increase the power load. Any PS that is rated less than 300 watts is not suitable. Even higher-capacity supplies that are not rated for the Pentium 4 and/or Athlon XP may not be acceptable. Even if the PS ratings are up to snuff, other factors come into play including its age. If the PS internal fan is bound up, clogged, or spinning slowly, the internal temperature will rise, potentially causing thermal shutdown and component damage.

Another is that some samples of the ECS K7S5A motherboard (and its clone, the PCChips M830LR) are subject to an interesting problem, one which I have direct experience with: the chipset voltage runs a bit high. One of the main symptom of this is the occasional loss of CMOS settings, as though the battery had gone bad; it can also lead to mis-read DDR RAM configuration, hard drive corruption, and other wierd problems. Here's the best link that I have found on this: http://www.geocities.com/mrathlon2000/chipset.html

The power supply is the more likely cause of your problem, but I include the other just in case.

I agree it probably is the power supply. I had the same problem with my generic PS. My advice is to get a good power supply like thermaltake and antec.

check your mother board