I'm not sold on the bad RAM; very possible that the hard drive is throwing craps. That could even be if the drive tests OK in Quick Test for whichever manufacturer of hard drive yours may be.
I see weird, failed installs of XP oftentimes when the HDD is dying. Seems nowadays, no matter what manufacturer of drive, I'm seeing drives slowing to a crawl inside of the two year mark. And surprisingly, having nothing to do with the typical "Windows Rot". I know this because clean installs on the same drive will STILL show odd, slow-as-molasses behavior.
For now, I'm sticking with Seagate, but even theirs are showing failure rates inside what I would call an acceptable time frame (to me, the 3 year mark).
My opinion is this; should you have to replace a hard drive on a customer's computer, that frickin' thing should last them until it's time for them to buy a new machine. In the real world, this ain't happenin' anymore, but it used to.