run the built-in diagnostic tool of xp...go to run > type dxdiag...under sound tab > test directsound..
if all tests run smoothly then your soundcard is not defective........i'm sure its a software conflict and troubleshooting it requires time and patience. try to use other apps other than you mentioned and see if its functional...but if you have some extra bucks i would prefer you to buy a pci to avoid the hassle.
note: before you conclude that it's defective try several ways to confirm that it is indeed defective.. gud luk troubleshooting!!!