Bleg: Skype is making me want to kick puppies

Hi, Internet:

I am trying to figure out why Skype won't work on my friend's computer. It's well within the system requirements Skype lists at the bottom of this page (the machine in question: Windows XP SP3, 512 MB RAM, 2.13 (-or-close) GHz processor) and it ran Skype fine until a couple of months ago. I don't know Skype well, but I have downloaded it and installed it and got it to run without drama on two other machines, one running XP, the other running Vista. I have also uninstalled and reinstalled Skype on the problematic machine -- trying both the latest stable version (4.2.0.169) and the v5 beta.

The built-in "Echo/Sound test" works fine.

The problematic machine is clean as far as I can tell -- fully up to date with Windows patches and drivers, and not infected according to several different scanning packages. All other apps work as expected. Skype seems to consume an inordinate amount of CPU on this machine -- typically 75-90% while on a Skype-to-Skype call, according to Windows Task Manager, and closely related, it is so sluggish that it appears to be frozen if you had normal expectations. I haven't actually been able to make it crash after a day of playing around with it, but I can see why my friend thinks it has.

One possible clue: it behaves better just running through the speakers than it does with a USB headset/mike plugged in. That is, in the latter case, text messages sent back and forth don't appear, the elapsed call time doesn't update, can't even get a response from something as simple as Help→About, etc. (Granted, with no mike in the former case, I'm not doing an apples-to-apples comparison.)

I can't find a conflict with drivers or ports, to the extent that I know how to check for those things. The various Windows admin tools all indicates "this device is working properly," including the one for the headset/mike.

[Added: The headset/mike unit is a "Skype-approved" one, according to what's stamped on it, and I believe my friend bought it through Skype, or at least, on Skype's recommendation. Also, due to the CPU usage symptom hint noted above, which appears whether or not I have the headset plugged in, I don't think this is the only problem, even though it seems to aggravate it.]

Any suggestions would be most welcome. TIA.