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Double entry in drivers.desc is really possible if you installed new version in different directory without uninstalling previous version but it shouldn't cause any problems because this value doesn't used anyway in VirtualDub (btw. I checked this by creating double entries myself and didn't see any problems in VD). As for your original problem (if you are talking about http://forums.virtualdub.org/index.php?act=ST&f=4&t=21319) than you should look for "vidc.x264" entry in HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Drivers32 and not in Wow6432Node which is for 32-bit apps. Also it should be x264vfw64.dll in for 64-bit case.
P.S. If you wouldn't provide any more info I will close this ticket at weekend because I don't really see any bug here (it is not task of installer to remove stray entries from registry).
Actually, it's not my problem, I'm just being emailed a lot about it
The 'not appearing in the codec list' is so far being seen on x64 os with both 32bit virtualdub and 32bit codec, and 64bit virtualdub and 64bit codec
Because the problem isn't here, and the users are having no success I've been trying to see what I can easily find out from them (as you know, getting information can be difficult)
Checking quartz.dll is registered, null values in the registry etc
So far, the double entry is the only thing that stands out as 'unusual'
Thanks for checking it on a pc - I'm at work atm, and my phone doesn't do virtualdub
Guess you can close the report
Most of the times it is really incorrect mix of 64-bit VD and 32-bit x264vfw or vice versa (not matter what such users say). Anyway ticket is closed as not bug. If you will be able reproduce bug than create new one (or reopen this).