What test gave you the hard drive failing result?

If your drive is flakey, it might test good, yet fail and crash Windows intermittantly - not uncommon. In other words, it happens to be working well at the time you test it, but fails every once in a while and crashes Windows.

300 watts should be okay for a P-4 or equal processor, so i doubt the new supply will help - unless the old PS was very low quality. Won't hurt either of course.

I'd replace the hard drive, if i were you.

BTW, i make a living as a PC consultant and deal with this stuff every day (and do web development/marketing).

Victoria is certainly right about being sure you have enough cooling airflow. You can buy fans or coolers specifically for your drive(s). And with lots of extra fans, you WILL need a beefier power supply, lol.

BTW, Victoria - if you get a new motherboard, it will probably have a dual core processor, no? That alone should cool things down a lot. I'm writing myself a spec for a new machine and i'm positive i won't need water cooling - though i'm looking at a fancy heat pipe cooler that replaces Intel's processor fan for the Core-Dual microprocessor.