If you go to the click here website you'll find hi-fi specialist speaker manufacturers such as Altec Lansing and Harmon Kardon do computer system speakers at remarkably low prices through to top of the range examples.
This comapany click here have a very large selection.
I bought a 2.1 Creative Labs iTrigue 3400's set from them - quality speakers especially when linked to a good sound card (Creative Labs XFi Music are pricey but worth the money)
If you invest in quality components you can easily and relatively cheaply (around £125 for speakers & sound card) turn your desktop into a hi-end sound system - mine sounds so good that I gave my HiFi away.
My local Superdrug (S Oxfordshire) had a 5.1 speaker system for £9.99 this morning. Not sure if it can be connected to a PC though, but with the correct adaptors shouldn't be a problem. Superdrug storefinder click here. As a precaution before purchasing ask if you can return them if not suitable/good enough/whatever.
Purchased a refurb Creative 2.1 set off eBay for £20 delivered some time ago - half the price of new ones at the time - still using them. PC speakers on eBay click here. G
An example of the type of speaker bargain you can pick up from eBay if there's someone in your town/city offering a good set with free collection in person click here. Bidding at £0.01 at the moment - will be interesting to see what it finally sells for. G