Video [player] issues

I ran into some problems a few days ago, reformatted my PC instead of working them out since I planned on doing it soon anyway, well my problem now is that NO video's so far as I can tell will work, no matter the format or player I use, I have tried WMP, Winamp, Divx, Quicktime and nothing works, it lags my PC severily and/or locks it up. Any ideas on what my problem is? I have never had this before and have reformatted more times than I can count.