Thank you for your post.
This is what I did to try and duplicate the problem.
On Windows Server 2003, I stopped FileMaker Server 10 and launched FileMaker Pro 10 Advanced. I opened up a test file that is set for Instant Web Publishing.
On My Dell Dimension running Windows XP, SP3, I downloaded Internet Explorer 7, installed it, restarting the computer, launched Internet Explorer 7, and entered the IP address of the Windows 2003 computer. The file appears in the Instant Web Publishing web page, and I'm able to select it and enter the Admin username with no password. The file opens without incident.
Is it just this one file that has the problem, or is it every file?
If it is every file, then it appears Internet Explorer 7 did not install correctly on your machine. I would download the installer run it to install Internet Explorer 7 again.
Do you have any other Windows XP machines that you can also try?