Previous to applying CU 24 (or later) to your 9.00 SP2 site, when you exported an HTML form report you could not save the file in MS Excel format (.xls).
This is intended functionality and was implemented in culmative update 24. It resolved a conflict with errors when opening the file in MS Excel 2010 and 2013.
Either use an appropriate application to open the .HTML file, or you can try to simply change the extension type to .XLS and attempt to open the file in MS Excel.
- Microsoft article on changing file extensions:
- List of site update release notes for 9.00 SP2.
- Product download page: