PC supplies are always labeled with their output voltages and capacities. They all vary. You want to check the spec on the supply for the +12 V line -- that's the one that should be 8 amps. It's easy to find supplies with tons of current capacity on the 5 volt line, but the 12V line is usually a lot less.
I don't favor the laptop batts either I've had too many go bad, and operating on them isn't simple. They will also likely be a mix of different cell types and capacities -- that's going to be hard to match up and figure out how to charge.
Not saying it can't be done -- just, might not work out well -- I just don't know and haven't tried it.
A lot of people swear by A123 batts or LiFe these days because of their safety record. Some people dissect electric drill battery packs and extract the cells. I haven't done that ether. Those cells have a different voltage than Lipos do, so I think you need more of them -- not sure.
Safety: you should charge lipos in a Lipo sack or bag -- these aren't expensive, and are fireproof. If you charge for only an hour, you can stay in attendance -- which is also important for safety. I've never had a problem in 3 years of flying, and don't know anybody else who has in my flying club either.
The main cause of lipo fires is setting the charge rate for a much larger battery pack, and then charging a smaller one at the same amperage without changing it to reflect the smaller battery. I've actually had a Nicad (Nimh actually) melt a plane's nose when being charged, so I don't think any battery is entirely safe. But almost always these things are due to human error, rather than a spontaneous problem. That can happen very rarely, but an appliance or aging house wiring can cause a fire spontaneously, too. For most people ( and here are millions flying R/C) there aren't any problems, other than "puffing" a cell because they flew too long without re-charging. That means one cell in a pack, usually gets gassy and the pack puffs up slightly. If that happens, time to dispose of it.
