Mike, a few things you can try:
Remove any COM add-ins you don't absolutely need. Do so by Office Button,
Excel Options, Addins. Adobe is a key one to remove. Bluetooth is another.
Lots of these will sneak back in. So check often
On the same panel, after removing the COM add-ins, select excel add-ins at
the bottom and click GO. Remove any VBA add-ins you don't absolutely need.
Minimize use of conditional formatting. If you have a lot of workbooks open,
close non-essential ones.
Close and restart Excel every hour or two.
Add more memory to your PC. I recommend a minimum of 3GB. 4 GB may be
slightly better. At a minimum, it makes all your programs more responsive.
Lots of debate on what is the minimum. Depends on the source! But more is
better.
Minimize the number of programs running on your PC. Get rid of Google
Desktop of you have it. Anything shown in the system tray is probably
running and taking up a share of PC memory, so eliminate what you can.
Ususally this is difficult as such programs hide how to remove themselves
from the system tray.
Consider a different anti-virus. I use TrendMicro (I do not own stock). I
dislike McAfee and Norton.
Robert Flanagan
http://www.add-ins.com
Productivity add-ins and downloadable books on VB macros for Excel