If the PC speakers are powered (either wall or battery), you should be able to plug them right into the headphone output of the MP3 player. Some speakers have funky connections, though. If it's just an 1/8 inch phone plug (like most small headphone plugs), it should work fine. If it's an RCA plug, just get an adapter to a stereo 1/8 inch phone plug. If it's some bizarre adapter, you might be out of luck.
Is there such a thing?
I have a Creative Zen player and this old (but perfectly working) set of PC speakers that I'd like to use.
Tired of googling this and seeing results for everything but what I'm looking for, so any help is appreciated.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic