Uninstalling Internet Explorer has been broken in some ways ever since IE7 came out.
The issue is that IE6 and IE7 (but not I believe IE8) were created as integral parts of Windows, so that you cannot install a later version without actually completely removing some core components of Windows. I *believe* that IE8 and especially the future Windows 7 use a more modular approach, so that some time in the future, downgrading your IE will no longer break Windows ; but that is not currently the case, because if you upgrade *from* IE7, some core components of Windows will basically be erased (the IE8 versions of these components are not IE7-friendly, but uninstalling IE8 will not necessarily restore the old IE7 versions).
I can see two things you can try :
1) Reinstall IE8 (this would be my personal recommendation)
2) Reinstall Vista
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Please bear in mind that IE upgrades are not designed to be optional upgrades, nor are they designed for downgradeability. The IE8 upgrade will eventually be distributed as a compulsory upgrade for all Windows versions.
It is not surprising if uninstalling a Beta version might create a mess, given that installing IE8 will affect some core elements of the operating System.