Hello Alberto!
I don't know if you get any error messages or what version you use, but you
can try the following steps .
1. Click on Start button then choose Run.
2. Enter the command "excel.exe /unregserver" (without quotation marks),
press OK
This forces Excel to unregister itself and then quit.
3. Click on the Start button then choose Run
4. Enter the command "excel.exe /regserver" (without quotation marks), press
OK
This forces Excel to register itself and then quit. Use this switch when you
want Excel to rewrite all its registry keys and reassociate itself with
Excel files, such as workbooks, charts, and others.
You can read more about start switches at:
http://support.microsoft.com/?id=291288
I hope this helps!
Stefan Hägglund
Microsoft