Did u have a lot of crashes? do u still have the old disk instaled?
if not maby the old disk had a higher power consumption and that was the problem.
maby it had a falty controler (on the disk) and was sending bad data 2 your cpu.
i added a new disk to a p2 a long time ago and it blew the power supply a 230w. a new power supply, 235 watt, made the system run and was so much more stable. the old one would crash when i ejcted a cd or just looked at the pc. i could not figer out what was up with it and my friends gave me such a hrd time for such an unstable pc.