It is not just a CSS problem.
Internet Exploder is displaying some of the code badly. All other browsers do not have that problem. So unless you want to have the forum display nicely in IE and show ill-formatted in any other browser, you have to provide different style information to different browsers.
You have the following choices:
- conditional comments: these work reliably and are perfectly suitable in this case
- JavaScript: unreliable at best
- UserAgent: unreliable and requires changing the web server's config
- CSS hacks: these are, like their name says, hacks - no alternative