While I know it may be too late, do
NOT install any codec packs. They more often than not will completely screw up your system and cause dll conflicts. The only one I know of that's actually reliable in most circumstances is the Combined Community Codec Pack (CCCP for short) - I know this because I know what components are in it, and in reality, the lion's share of playback it provides is actually handled by ffdshow, which is something I use often. In reality, you could just skip installing the CCCP and use ffdshow instead - just take a look at the configuration first and select the formats you need it to handle (ffdshow uses libavcodec to handle decoding of many different formats; libavcodec is the same decoding library used by VLC player and mplayer - ffdshow is simply integrated into DirectShow and Video for Windows so the video can play in Windows Media Player or software like VirtualDub). You also might want to check out Gspot - it's an analyzing tool that will tell you exactly what format the video is (AVI is not a format - it's a container that can store any number of different formats).
For ffdshow, see this page on Sourceforge (I recommend using clsid's nightly builds although you might feel better if you use the beta versions - they're older but generally regarded as more stable):
http://sourceforge.net/project/showfiles.php?group_id=173941
Two other possibilities - you exported in DV, which will require a proper decoder (ffdshow can handle DV just like it can handle DivX, XviD, H.264, etc., although I don't know if it's restricted to Type 1 or Type 2 DV), or if you exported as Uncompressed/Full Frame video, then the AVI's index may be corrupted and it'll need to be fixed. VirtualDub has options to reconstruct AVI indexes or to convert to other formats, usually in AVI (MKV and OGM are supported in VirtualDubMod, an older fork of the project that has kind of died development-wise; I still use it, though, and it's still solid for what I use it for, but more involved tasks are best handled elsewhere - and that usually includes the main branch of VirtualDub also).
My recommendation is to get ahold of HuffYUV and Lagarith, which are two lossless codecs that can reduce filesize a lot over the Uncompressed copy, but not nearly as much as lossy codecs like XviD, MPEG-1 or -2, H.264, etc. Mainly, HuffYUV and Lagarith are good for intermediate compression to preserve quality, and to store in backup as master copies (they also good to use during the editing process itself, because they don't suffer from generational decay). There's a limitation, though, in standard HuffYUV doesn't support the YV12 colorspace, and Lagarith does. But, Lagarith may not always be desirable to use due to hardware strength not being up to the task or because some non-linear editing software are known not to play nice with it. ffdshow (again, haha) does have a YV12 implementation of HuffYUV in its VFW interface, and it's very reliable.
