Microsoft Advice:

Uninstall Internet Explorer 8 (and some other stuff.

1. To continue receiving WinXP updates, you MUST INSTALL Service Pack 3 (SP3).

2. If you installed SP3 after either IE7 or IE8 was installed, you cannot remove IE7 or IE8. (Its unclear whether an "upgrade" of an IE7 that was "unremovable" sould make the IE8 upgrade unremovable.)

3. If this is a problem, you should be able to uninstall SP3, after which you should be able to uninstall IE7 or IE8, after which you can reinstall SP3, after which you can reinstall either IE7 or IE8 so that a "backup" removal can be done.

It's uncertain whether this is your problem, but the article linked gives more complete discussion that may help you decide.

The article also includes links to Microsoft instructions for how to uninstall SP3 and both IE7 and IE8 in WinXP.

John