Not sure if it supports the functionality needed for removing the noise, but Doug Oade recently mentioned over on his forum this FREE audio editor:
AudacityWhile I use CEP2.0, I'm gonna give
Audacity a go one of these days just for kicks.
FWIW, I've performed noise reduction similar to your case in Cool Edit 200/Pro. If you have a silent section of recording where the only or primary sound source is the offending hiss, you can take a snapshot of the noise and then remove the noise (based on the snapshot) from the rest of the recording. Lots of options for tweaking exactly how this is done. Exact functionality depends on your audio editor. Basically the same idea as what the last two posters mentioned. Not sure how configurable the XP SOUND SOAP is, as I've never used it.
I will tell you that it took me damn near forever to get it to a point where it sounded good - and I still wasn't completely satisfied. It's far easier to screw it up even worse than to make it better. LOTS of trial and error by ear, ad nauseum. Try it, you'll see what I mean.