Browser/WordPress/Theme Incompatibility?

Trying to get a new site up and running and would like to use the Twenty Eleven theme.

Upgraded to WP v.3.2 to solve initial compatibility issues, but now finding display compatibility problems between browsers…

IE 7 displays fine

IE 8 displays problems with menu bar/search bar when “compatibility mode” is turned on, and problems with the photo plugin I have installed in the sidebar when it is turned off.

My dashboard tells me that I’m running an “insecure” version of IE and that I should upgrade. It takes me to the IE9 download page.

IE 9 – others report that IE9 displays fine HOWEVER – my computer is running XP and at this time an OS upgrade to Vista or Windows 7 is out of the question, so an upgrade to IE9 is not possible. Not to mention the large number of folks out there who will still be using IE8 on their own computers when viewing my site.

Firefox 5 is also problematic – it correctly displays the menu/search bar in all cases, but on some computers (and even on the same computer at different times – random!) improperly sizes the photos in the plugin in the sidebar.

I really am at a loss as to what to do – is the incompatibility with WordPress, IE, Firefox, the theme?

Will a different theme help the situation?
Should I roll back to WP 3.1.4?

I’m not super versed in WP and I do my best to stay away from code – I’m not a web designer by any means. Point & click options work best, but any advice you have or if you know even what general direction to point me in – I’d be grateful.

Thank you!
Cathy