I just found out about the Yahoo UI Reset CSS tool/file and I\'m using it on my website. On my machine I have Internet Explorer 7 and Firefox 3.01 and my webpage looks the same
A reset CSS--of which YUI's is only one example--aim to remove browser differences in default CSS properties for HTML elements, like some browsers will have a default 8 pixel padding on the html element (it was a surprise to me to find the html element was even styled when I first found out).
Your IE6 issues are entirely separate to that. Without examples of the site itself it's impossible to say why this might be but the obvious guess is that you're relying on reasonably advanced CSS features (eg a > b selectors, :hover on non-anchors, certain positioning schemes and so on).