Then clearly it is not an Audacity issue.
Something like this is almost certainly to do with drivers somewhere.
Have you installed any “virtual” audio drivers for example in a recording program? If so try uninstalling that program. If necessary, you could use System Restore to go back to a restore point before the problem started.
Otherwise I suggest you uninstall the Realtek audio device from Device Manager. Do not restart the computer or it will probably attempt to reinstall the same drivers. If you have a drivers CD that came with the computer, install the audio drivers from there then reboot the computer.
If you have no drivers CD you will have to go online, which can be very dangerous on XP. I suggest you follow audacity not saving - #3 by Gale_Andrews at the earliest opportunity. Go to the web site of the computer manufacturer (such as Dell or HP) or if you don’t have a branded computer, go the web site of the motherboard manufacturer. Download and install the Windows XP audio drivers for your exact computer model or motherboard model. Do this even if Windows says your drivers are up-to-date.
Gale