Can't you find it under
[Start] >
[Programms] > etc. ?
(Shows 2 remove options: bundle and codec)
(picture concerns 5.2.1. version of codec)
Depending on version of BS.Player you can use its filtermanagement:
(rightclick > Options > Preferences > Filter management > [Filter manager] > Video decompressors
to unregister filter.
Otherwise through
[Start] >
[Run] > and then:
%windir%\system32\regsvr32/u [full path to .ax-file]\[name of .ax-file]
Which means in your case:
C:\Windows\system32\regsvr32/u C:\Windows\System32\Divxdec.ax
For Win9x systems:
%windir%\system\regsvr32/u [full path to .ax-file]\[name of .ax-file]
Further there are also several possibilities offered by Windows (the following indications are for XP):
Either rightclick on "My computer" > Properties > [Hardware] > ....
or go from System configuration screen > [System] > [Hardware] > ....
... then: click on first option > drivers for sound, video and games > video codecs > [Properties]
(Note "labels" are translated from a Dutch OS, so they may differ)
Yet another possibility is:
System configuration screen: > Sounds and audioappliances > Hardware > Videocodecs > [Properties] (2x) then you can select codec and remove it.