It is probably the Fast Index service running in the background that is slowing your PC. See:
http://support.microsoft.com/default.aspx?scid=KB;en-us;q199787
A few other things you could do to speed up your Excel application is to remove the frills:
On the Excel Menu Tools, Customize Tab
Uncheck Menus show recently used commands first, it is a stupid option that is very counterproductive and it uses memory.
On the Excel Menu Tools, Options General Tab
Uncheck provide feedback with sound.
and on the Edit tab uncheck provide feedback with animation. Click ok exit Excel and see if that helps.
As a LAST resort that I would only recommend if you have alot of formulas (5000 or more) or if you have formulas that reference other worksheet files you could switch to manual calculation and hit F9 to force a manual calculation. This can be done from the from the Excel menu Tools options Calculation tab.
Hope this helps
Charles