Okay, here we go:

I got Photoshop Elements 4.0 the other day, and I finally went to install it on my computer. I installed it, everything seemed to be working fine, blah, blah, blah.

But there's a problem. When I click on the Photoshop icon, I do get the welcome screen. However, when I go to view photos and get started, as the screen, which tells you that it's loading plug-ins and etc, appears, this pops up: "The interface is unknown." When I click "ok", everything disappears.

I have Windows XP with the Service Pack 2 and everything. I've tried fixing installation (EDIT: yes, i've tried reinstalling it). I've run virus scans and such. (EDIT: Restarted it everytime I change something.) Nothing helps.

Does anyone know what might be wrong? (Google didn't help.)

"Fixing Installation" does that mean you reinstalled it, if not, then you should try that.
Or maybe restart the computer.

delete completely like everything and then install it again and restart it also.

Like using a katana to trim a loose thread...