I've had this occur twice now, the first time was with Powerpoint, and today
with Excel.

The first time I did a system restore that worked!

This time I sat and used the old grey matter!

I realised that the cause could be registry related, i.e. one of the
assosciated registry keys was corrupted.

I tried the following:

Close Office Application affected, excel/ powerpoint/ word etc.

Click the Start button and choose Run.

In the command box type the following:

excel /unregister

Click th OK button

Now type:

excel /register

Click th OK button

now check the operation was sucess ful by opening the Office program
affected.

Notes:
1. there should be a space between the Office program name e.g. excel and
the /.

2. for other Office programs affected by this problem simply substitute:
word/outlook/powerpoint etc.

Good luck.
Henry.