I'm still not sure exactly how the file association got messed up though. It seems the problem is caused by the chipamp plugin (or so someone claimed in one of the forums,) which i know i installed on my old computer and i'm pretty sure i installed on this computer as well (although i can't actually find it in any of the lists of plugins in winamp preferences.) However if so i installed it within a couple days of when i got the computer. So why did the bug wait until now to suddenly pop up? I haven't updated winamp any time recently as far as i can remember, so what changed? I did install some steam games and the PS3 controller driver on saturday, but everything was working fine until monday evening, so i don't see how it could be related to that either.
Speaking of which, for anyone else who wants to try out using your PS3 controller on your PC (and is convinced by the argument that it can't have been responsible for my internet problems =) i found two popular methods.
One uses the "MotionInJoy" drivers, with detailed instructions on how to install it here
The other uses a "SIXAXIS driver" with instructions here (among several places.)
I tried the MotionInJoy driver since that was the first method i discovered. The setup is a little finicky, and every so often (after turning off or unplugging the controller) i have to kill the driver, restart it and reconfigure it, but it definitely works well when you've got it configured properly. I haven't tried the other method yet to see if it's any easier/more stable.