If it happens with headphones and speakers, it's nothing wrong with the speakers, interference, etc.
Probably a ****ty sound card that's failing. If your computer is under warranty, make them replace the motherboard (almost all computer's sold in the last 5 years that weren't sold as very high end models have sound cards built into the motherboard).
If not, you can by a cheap replacement sound card and disable the onboard one.
For example: http://www.newegg.com/Product/Produc...82E16829265001
Of course, unless you know what you are doing with computers, find a friend that does (and buy them a damn six-pack or something for helping!) =)
or you can buy a USB one you don't have to crack the case to install:
I've never used one of those, so for all I know they are complete ****, but *shrug*