Excel formatting

I've used Excel for many years and currently Excel from Office XP
Professional. I have no problems with the programme on my main computer but
I am now unable to format any cells on my Laptop application. Clicking rows
also puts shading into "black". There have been other difficulties with data
input also. I have re-loaded the programme with the original discs but the
problem remains and occurs with any spreadsheet I work on, although these are
fine on the other computer.

Help would be appreciated.

Dr John Burgess