I have heard that Vista-formated partitions are incompatible with XP. But not 100% sure about it. So you might need to backup again, and format your primary partition + do clean install.
Personally, I have managed to get all the codecs/software I need even for Vista 64bit, which is harder. You could also try to get VLC video player, which is free and and has codecs compiled in. I use it to view movies that media player has problems with. Also, installing codecs is quite good way to get PC infected with some crap.
Edited by Giedrius M, 04 July 2007 - 10:33 AM.