@zeus
it would be in your best interest if you post what speakers (brand and model) you will be connecting to the pc or at minimum what wattage output and impedance they are rated for.
do you need a receiver? no. this can be handy if you have multiple sources or are needing a stronger amplifier as sometimes its cheaper just to buy a receiver than a strong amplifier but we will cross that road if we need to once you list your model numbers. in most cases you can get a nice little amplifier to power bookshelf speakers just fine.
a nice little amp would be the dta-120. we have it hooked up in the other room with a pair of HK thirty bookshelf speakers and it is excellent.
the lepai 2020+ is a good cheap amp if you have fairly low power bookshelves although if you need more output its going to start distorting fairly early. you get what you pay for, remember that.
just want an amp? hook up to it via a standard 3.5mm to rca cable (for rca input model. or 3.5mm to 3.5mm for ones with an aux in) out the rear of your pc (either onboard audio outputs or soundcard). if you have poor qualityi onboard audio (static, buzzing, etc) you can get a cheap external dac or internal soundcard.
lets first start with A) what equipment you have and B) if the pc will be the only input device.