Sound Pops, Cracks and Drags

I can hear audio, but it hisses, pops, sounds
scratchy, slows or sometimes drops in and out. This
happens at any time but is very much pronounced when I
open another application such as Outlook Express. This
happens in any playback application including WMP, MMJB,
Real, audio CD's. In an attempt to try to fix the
problem, I installed a Philips PSC604 Sound Card. Well,
it sounds a little better but it didn't not cure any of
the problem.

My computer is a Medion 1294, P4, 2.66 GHz, Windows XP
Home, with plenty of space left on everything.

Thanks for any ideas.

Gary