Very slow to make changes in Excel 2007 sp2
 

I have a machine with Excel 2007 sp2 that when making changes takes a very
long time... so if I bold or un-bold a cell it will take up to 30 seconds...
once the change is made I can undo/redo and it is instant, but if I un-bold
the same cell instead of undo it again takes up to 30 seconds.

Is there a procedure to try and diagnose what is causing this?

Thanks