Thank you seavixen.
I think you're probably right about my graphics adapter - it's a Matrox G450 MMS Quad Graphics 128MB card. I run 4 monitors for daytrading. It's quite a few years old - at a guess I'd say 10yrs although I bought it second hand. Matrox don't supply a driver
for Windows 7 for this card although they do have drivers
for XP and Vista which work fine on Win 7.
My system specs are:
Dell Studio 450
Intel Core 2 Quad CPU Q8200 2.33GHz
Win 7 Ultimate 64bit
I have actually found a solution since my first post - I went to Control Panel - System and Settings - System - Advanced System Settings - Advanced - Performance Settings - Visual Effects and then I chose "Adjust for best performance". This seems to disable the Aero feature and puts a classic grey theme on the windows. Excel and Word are a lot quicker now. Bit of a shame as I like the look of Windows 7 Aero but performance is more important for me.
I guess the only solution is to buy a more recent quad graphics card that can cope with Windows 7 Aero?