The power switches are usually just simple 'push-to-make' generic types on most modern PC's and can be replaced easily enough with parts available from your local electronics retailer (Maplin/Radio Shack etc.). They may be glued into the case though. You'll need to solder the wires back on to the new switch of course. The button part is normally just a plastic cover in most cases and is more to match the casing than to switch anything. It'll have a bit sticking out at the back that connects to the actual switch.
<safety> NOTE: Take care when doing anything involving electricity and make sure you unplug (not just switch off) from the main power point BEFORE attempting any repairs. </safety>
Or you could take it to a repair shop (and pay a small fortune) and have it done for you.
If I can't be a good example, I'll just have to be a terrible warning...