If it's a model 5000US, you do have an older tower. You can go to www.crucial.com and download their scanner for a custom check. You don't have to buy the RAM from them, though.

Your machine takes PC133 RAM, with a max of 256 MB per slot (according to my manual check at crucial.com, your tower has 3 RAM slots). This stuff is expensive to buy, but if you know anyone with an older computer, or are on Freecycle, you might be able to get this for free!

It seems you were able to get into Safe Mode, so you might want to back everything up and reinstall Windows XP. I haven't tried backing up data in Safe Mode, so someone else here will need to see if it works.