Do not install Internet Explorer 8 Beta. There is no uninstall.

this information may help you

http://www.zdnetasia.com/news/software/0,39044164,62045585,00.htm

I hate Microsoft. MSN Explorer and IE are disasters in the making.
Every time there's a change or upgrade, I suffer permanent
consequences. MS does not understand the meaning of the word
progress. The only thing they care about is prestige regardless of
whether or not anything works. Thanks for the heads up on IE8. The
last thing I need is another problem.

What happens when you try the following:

1. Click the Start Button and select Control Panel.
2. Select Add or Remove Programs.
3. Find and select Windows Internet Explorer 8 from the list of
available applications.
4. Click the Remove button.
5. Restart your system.

That's for XP, it's "View installed updates" on Vista.

You should then be able to reinstall IE7.

I take it that one of these steps didn't work?