I have no idea why this is... I can't give many details, other than my CSS file, but why does my menu go all weird on everybody's computers, except mine?

http://www.execulink.com/~kayes/phys...animations.htm
BTW tell me if you like the page

What menu ? I don't even see a menu in Firefox 3 and don't see it in your code either.
However, I do see you're using an XHTML 1.1 doctype, which is NOT widely supported. Better to drop it back to 1.0 for better reliability cross-browser.

By the way, you really should only have 1 <h1> to a page

I've decided to take it out... Just forget about all the problems it gave me...

I also just use the <h1>'s as titles.

H1's as titles is fine, but semantically, an H1 is your MAIN title, any subsequent headings should be h2, h3, h4, etc. - following a proper OUTLINE structure. In other words, 1 main topic per page.

Ah.

I could use my h4 (the underlined one) to make the MAIN title, and h1 to make all SUBSEQUENT titles...

You could, but the search engines DO pay attention to heading hierarchy. Using an h4 for your main title would be saying that that tile is not the main subject of the page and less important.

SEMANTICALLY, use a proper heading structure where h1 is highest, and following sub-headings are in order -h2 thru h5, 5 being the LOWEST level of importance.