So... I just wouldn't accept that another beloved game of mine wouldn't work on Vista, so I tampered with some settings and gasp... I got it to work. I signed up just to let people know how to get it to work.
Well first I went to C:/Program Files/Enterbrain/RPGXP. If you have it installed in a different folder or drive then go to the RPGXP folder and whatnot. Then,
click right-click properties on the RPGXP.exe file. Go to the compatibility tab, and then click the box that says 'run this application as administrator'. Click apply, then close.
Then you must right click on the file PSIkey.dll and go to properties. Then click the security tab and go to advanced. You'll notice that the Users(pavilion/users) or whatever computer name you may have doesn't say full control. Well... highlight the Users(pavilion/users) and click the edit button. Then in the new window, highlight it again and click edit. In the large permissions list, tick the first box that says 'full control', then click ok.
Be sure that the box that says 'include inheritable permissions from this object's parent' in the window you just returned to is ticked. Click ok, then ok again, and then ok once more and RPGXP shouls work.
I'm assuming that the people with that serial number error have the trial like me. Now, excuse me while I absorb myself into RPGXP for the next 107.25 hours...