Hello John,
have you tried to re-install Microsoft Streets & Trips 2007 again since encountering this problem?

Also you could try the default program settings for your Electrical Estimator file: right click the file and select 'open with..' and then choose the 'select default program' option, browse your program files folder and then select the Electrical Estimator program.

Make sure you check the box at the bottom 'Always use the selected program....'