Arakura, since you apparently have not already done so, I'd recommend acquiring either the K-Lite or CCCP codec pack (NOT both, multiple codec packs often cause conflicts!). They not only come with nearly every codec you could envision (for both encoding and decoding), but both also come with MPC-HC (Media Player Classic - Home Cinema), one of the better players out there. MPC-HC + ffdshow will play almost everything.
I recommend staying away from VLC Player, btw, which many people like, but is buggy as hell + has terrible scaling algorithms.