Is this an IE bug?

I have a page that uses a menuing sytem created using CSS. In
IE(5,5.5 & 6), if you manually resize the page, the menu becomes
unreadable as the borders of the links are 'too big'. This problem
doesn't occur in Opera (7.51), Netscape (6 & 7.01) or Mozilla (1.7)

Does anyone have any thoughts?

http://www.geocities.com/emmaruthuk/test/HTMLPage1.htm

The source code has been run through the validator, and comes out
valid for XHTML1.0 Strict, the CSS validates also.

Emma