You must only put the supported Win32 DLL files in this ("DLL") folder
Get dll's from here: http://www1.mplayerhq.hu/MPlayer/releases/codecs/
(then extract/uncompress them an put here into "/mplayer/codecs/")

Currently (20 August 2004) the only XBMC supported DLL's are:

WMV9/WMA9:

wmadmod.dll	(File version: 9.0.0.2926)	410,216 bytes
wmsdmod.dll	(File version: 9.0.0.2980)	760,968 bytes
wmspdmod.dll	(File version: 9.0.0.2926)	486,504 bytes
wmvdmod.dll	(File version: 9.0.0.2926)	807,528 bytes


RealMedia (RealAudio/RealVideo):

atrc3260.dll	(File version: 6.0.1.1651)	 73,766 bytes	ATRAC3 audio (Sony ATRAC3 codec)
cook3260.dll   	(File version: 6.0.8.2523)	 65,575 bytes	COOK audio
drv23260.dll   	(File version: 6.0.7.3928)	176,165 bytes	RV20(G2) video(use force frame rate option)
drv33260.dll   	(File version: 6.0.7.4085)	208,935 bytes	RV30(RV8) video
drv43260.dll   	(File version: 6.0.7.2389)	217,127 bytes	RV40(RV9) video
sipr3260.dll   	(File version: 6.0.7.3922)	102,439 bytes	SIPR audio(ACELP-NET Voice Codec)
(Note! dnet audio and RV10/13/20 video/audio are support by native codecs)


VP4/VP5/VP6:

vp4vfw.dll	(File version: 4.0.20.24)	466,944 bytes   On2 VP4 video codec
vp5vfw.dll	(File version: 5.0.15.1)	372,736 bytes   On2 VP5 video codec
vp6vfw.dll	(File version: 6.2.0.10)	438,272 bytes   On2 VP6 video codec


QuickTime Audio:

qtmlClient.dll			(File version: )		225,280 bytes
QuickTime.qts			(File version: 5.0.2.15)	4,206,080 bytes
QuickTime.qts.htcfg		(File version: )		189,958 bytes
QuickTimeEssentials.qtx		(File version: 5.0.2.15)	532,992 bytes
QuickTimeInternetExtras.qtx	(File version: 5.0.1.10)	830,464 bytes