Jim,
I've already got everything turned off in MC10, and everything on in WMP9. I also checked the associations in Windows Explorer, and it shows (properly) that WMV is associated with WMP (it also shows WMA and every WMP type properly associated.)
I don't have any WMV files downloaded to test that on, but when I select MP3s on my hard drive, they open in WMP with no problem.
As soon as I try to stream any WMP audio or video, though, it will not open in WMP... it keeps going back to MC10. Mind you, when I open the tools>options menu in MC, everything is still uncheck (except for .MPL playlists, which is automatically checked every time I open the menu.)
Here is a paste from the Help>System Info tab:
Media Center Registered 10.0.42 -- C:\Program Files\J River\Media Center\
Microsoft Windows XP 5.1 Service Pack 1 (Build 2600)
Intel Pentium 4 2425 MHz MMX / Memory: Total - 523 MB, Free - 225 MB
Internet Explorer: 6.0.2800.1106 / ComCtl32.dll: 5.82 (xpsp1.020828-1920) / Shlwapi.dll: 6.00.2800.1276 / Shell32.dll: 6.00.2800.1233 (xpsp2.030604-1804) / wnaspi32.dll: N/A
Ripping / Drive L: Copy mode:ModeSecure CD Type:Auto Read speed:Max
Drive M: Copy mode:ModeSecure CD Type:Auto Read speed:Max
Drive N: Copy mode:Normal CD Type:Auto Read speed:Max
Drive O: Copy mode:Normal CD Type:Auto Read speed:Max
Digital playback: Yes / Use YADB: Yes / Get cover art: Yes / Calc replay gain: Yes / Copy volume: 32767
Eject after ripping: No / Play sound after ripping: No
Burning / Drive M: TDK CDRW4800B Addr: 2:1:0 Speed:48 MaxSpeed:48 BurnProof:Yes
Test mode: No / Eject after writing: Yes / Direct decoding: Yes / Write CD-Text: No
Use playback settings: No / Normalization: None