"all of my users are complaining that the HTML version no longer works to compose an email."
"Users that run a higher resolution have the option of using AJAX but, it can be unbearably slow using IE6."
By that we know you're talking about the standard client, but I don't know if you're composing in HTML mode or TEXT mode?
Bug 28361 - HTML Client : can't type in msg body using IE & text mode (I can repo that on IE7.)
So if that's what you're hitting:
A) If they're set to Text mode change them to HTML compose.
OR
B) If they're aliment on text mode, setup a signature, then make sure it's set to include that in the persona (accounts tab).
For the Advanced client we test IE6SP2 as listed in the
system requirements, but with Win2k well that's not possible if there's no IE6SP2. Bug 28361 simply sprung up for the Standard client, hope that's what you're hitting as there's 2 quick workarounds.