"using a player that did not need codecs" - sorry, not reading carefully enough

Have you tried JetAudio?

I use Jetaudio 6 but it now seems Jetaudio 7 is available.

It is free!

click here

A friend of mine had a similar problem a while ago - very low resolution all the time and sometimes not completely rendering - like a half-finished Lautrec painting.. It turned out to be a graphics card/driver problem. Though we never figured out why it started doing this (possibly corrupt drivers), we did fix it by removing then reinstalling the Graphic card (I think we may have had to change which PCI slot it was in, but not certain of that - bear it in mind, later). I am assuming you have a PCI card, and not AGP card or OnBoard graphics. For OnBoard, reinstalling drivers is the only option, aside from buying a card.
You haven't given a link to your earlier discussion so, I don't know what you've tried so far but I'll go from first principles.. I.E.-First try just reinstalling the drivers: Check manufacturer's website for latest drivers first ('google' their name).Once at their website look for support or downloads and, if it is an old card/motherboard you may have to check Archive or Legacy products. 'Might also be worth checking the site's FAQ or Forum for 'Known Issues'.
Remove the graphic card in DEVICE MANAGER (go into device manager[or whatver it's called in W2k] and select the graphic card, then click REMOVE, click OKs to exit and RESTART.
NB-To simply reinstall drivers, you DO NOT need to physically remove the card.
When the system restarts it should say "found new hardware" and start looking for drivers - allow it to look for the correct drivers - unless this card requires manual installation of drivers.. I suggest you check its documentation if you haven't done it before.
Once installed, you will probably be asked to Restart again - do so, and let us know how you get on.
I'll check back a bit later - I have to go out for a short while.
Hope this helps.

Rab