Most computer power supplies aren't regulated so you need to have a small load on them to stabilise the output voltage / current.

I personally wouldn't recommend simply plugging your charger into one without doing the mods you see in various forums. It may work, but it could be risky, both to you and your charger!

Safety disclaimer:

If you are interested in making your own - remember that it is possible to do all the wiring and mods without actually opening the psu - so it's quite safe that way, but if you dont know what you're doing then I suggest you don't attempt it!