Depending on the size of the speakers your PC's audio adapter's built-in amplifiers may be enough. If they are larger/higher wattage speakers (say from a home stereo) then you'll need some kind of amplifier. You'd set it as being line-out on the PC, and then the amp will amplify it for the speakers.
You'll want to check the speakers for markings to try and determine their OHM and amperage ratings and find an amp that agrees, so you don't end up over-driving the speakers.
You probably aren't going to need more than a 40-80 watt amp to power those speakers, and they can be had in the US and Canada for less than $100 in many places.
As an example here's a 40 watt stereo amp from Pyle, that has an MSRP of around $100; overstock.com (for example) has them listed for ~$60-70 right now.
You should be able to find something similar at Radio Shack, or an audio (car or home) store, or perhaps even check out a local surplus store.
You can also get build-it-yourself kits for low power amps from various electronics stores on-line.