IE6 is a pain. You could always look at the livetest site to see if it's an issue with ubercart and your theme, or just ubercart. Either way, IE 6 is the devil, and it will take a little bit of playing around to figure out what's causing it. I always start by getting a site that looks good in firefox. Fire bug is great for theming. Next, I make sure it passes validation, then I check IE7, and finally fight with IE6. Try using
the IE web developer toolbar, it helps a little, but it's nothing like firebug.

also, it helps to use conditional comments