Hi
Please try the methods below to see if this can help resolve the issue:
1. Open the site, for example http://mossportal/site/default.aspx
2. Then add "?contents=1" to the end of url, like this: http://mossportal/site/default.aspx?contents=1
3. You have a Web Part Page Maintenance and if you have some web part error delete it
4. Open the page with SharePoint Designer to see if it works
If this doesn’t help, could you paste the detailed message which asked to install FrontPage extension here? What’s more, check the event viewer to see if there is any clue there.
For more information, please refer to: http://umakanthn.blogspot.com/2008/10/server-could-not-complete-your-request.html
Let me know the result, thanks.