Cimon
Said..
The technique is to uninstall IE8 with Add/Remove Programs and it should leave you with IE7 (or 6 or wherever you came from). BUT if you had beta 1 or beta 2 installed on XP SP3 and you installed IE8 RC1 over the top of it, it won't come off. There is a warning on the Microsoft download site about it.
Did you try putting IE8 in compatibility mode? that is supposed to handle the non standard compliant websites in the same way as IE7 did.