I think WMP 7.1 worked. However, I can only roll back to WMP 9 (which don't work)! Can I re-install 7.1 over 9?

I dl'ed & installed foobar2000 which worked (as does Nero Media Player). Media Player Classic (dl'ed exe file only) gave me the same result as WMP 9 & 10 (no good).

If I can't rectify the problem I'm more than willing to avoid using WMP. However, I'm still curious as to know why it don't work! Something to do with codecs perhaps?