Are you saying that you installed the Dell version of XP on a non-Dell computer? If so, that is your problem. You can't install Dell's XP on a non-Dell PC. I tried doing that with my pc and everything was screwed up, I ended up buying my own copy of XP home. I was also unsuccessful at installing Dell's XP on an HP laptop and an HP desktop.
So if this is what you have tried to do, it may work for some things but I can almost guarantee that it's causing you those problems.
If you ran Memtest for at least 4hrs, and it gave you errors...then you answered your own question in your previous post..lol. It means your RAM is bad. You'll need to get new RAM. RMA the old RAM, if possible.