Problem solved!
A few things I've found:
Opening WMV's in WMP 6.4 or MPC work fine. WMP 9 crashes.
WMP 9 is now the default media player plugin for web browsers; you can tell by the skinned interface in embedded movies. I'm fairly certain WMP 6.4 (mplayer2.exe) used to be the default plugin for web. In any case, this would explain the crashing of the web browser.
Windows Media Player is considered a vital part of the OS. If you delete or replace wmplayer.exe or mplayer2.exe, Windows (XP) will automatically restore the files completely silently from your system32\dllcache folder. Note that if you delete the file from the dllcache, there are a number of other places that XP will look to copy the file from, including paths like WINDOWS\$NtServicePackUninstall$ that hold the the previous version of WMP, in my case WMP 8. If Windows decides to restore this file, it will break WMP altogether as you are using a WMP8 exe in your WMP9 install. This will give you the fun error message "An internal application error has occurred". In some ways it is better than the outright crash. :happy:
Even if you delete all the backup versions of the files (Windows will let you know when this happens and prompt you for your CD - how nice =P), replacing the exe's with MP Classic doesn't seem to do anything to the browser. I could have sworn I'd heard of people doing that before, or possibly I'm just hallucinating.
Anyway, here's the (a?) solution. I searched a long time, but couldn't find anyone who solved their problem. However a common suggestion was to remove "illegal" codec packs. Now, the phrase "illegal codec" made me doubt if their intelligence was any higher than a monkey's, so I just ignored them for a while. Eventually I did try uninstalling Matroska Pack (not illegal), and it fixed it. It definitely seems to be a codec conflict that WMP9 can't handle. The pack contained MKV splitter, Realmedia splitter, ffdshow, VSFilter, CoreAAC, CoreVorbis, CoreFLAC, AC3Filter, MatrixMixer and Morgan Stream Switcher. Some of these are kind of useful, so I'd like to find out exactly which one causes the problems.