Welcome to the forums... try using the search feature here. You'll turn up a lot of threads about this.
I think, like speakers, "the best sound card" question is more subjective than anything else. For what it's worth, I've used both (currently using an Audigy) and here's what I think between the 2.
Audigy --> better sound positioning during game play
Santa Cruz --> better at audio playback
both are great for "hardware" AC-3 playback. If you're going for the cheapest, instead of the Audigy OEM, I would just go ahead and get the Santa Cruz.
EDIT: Using WinXP Pro with my Audigy... no problems. Just have to install correctly!! And also, which ever card you decide to get, I'm assuming the Compaq you're using has onboard sound. In that case, make sure you disable the onboard audio support through the BIOS (if that's not possible, just disable it through Windows) before you install your new card.