Installing Microsoft OS Beta software is done at your own peril.
IE8 Beta 2 was almost unusable for me (due to habits, like cutting the address bar, it locked up. A known bug). Auto Complete couldn't be turned off either or it crashed completely!
I had to uninstall it, luckily IE7 works fine again for me - reading around the net, others had some problems "rolling back".
I am going to wait for the proper release of SP2.