Not sure what version of software you're using but check this link: KTV01C_download
Also, a codec pack might resolve the issue.
I generally use the K-Lite Mega packs as having more than on codec program can cause conflicts.
However I ran into a brand new component pack for Windows 7 64-bit: Win7x64 Components 1.2.2 Final
Win7x64 Components will enable MediaCenter on x64 installations to play all your media files.
Windows 7 x64 uses the 32bit player as the system default. The installed components of this release include shortcuts to allow you to change the system's default player between the 32bit and 64bit Windows Media Player(s).