How do you check your cross browser compatibility for css?

I had designing a site for a client and just prior to making it live they said there was a problem with repeating header all the way down the screen.

They are running AOL/Windows XP on a laptop - which should be IE6

I have tested on my computer for firefox/ie6&7 and can't replicate the problem.

I have managed to use browsershots dot org which shows the problem for IE6/windows 2000, but it does not refresh when I make changes to the stylesheet, so I am working blind.

Any ideas?

This site has to go live Monday morning to comply with grant funding application. I may explode by then