laggy spreadsheet

[Curt]

I have office xp pro. A spreadsheet i use every month
for business became very laggy when i input or change
info. If i go back to an older spreadsheet (same
spreadsheet) it works fine. I cant figure out what i did.
Its a large (very important) spreadsheet and i cant start
over from several months ago. I have checked and
installed all office updates.

thanks