my excel file use 100 % CPU usage

when ever i try to do some changes on my few excle files it used 100% usage of my cpu. i have cheked my memory but just get used 191mb. everything works fine but this the most imp file. i have try to open this file on the other computer but the same porblem.the file size is just 865kb which is noting to concern.but why it used 100% usage of cpu? it takes too long to make any changes to a file.
system Info:-
cpu: 2.4ghz Intel
ram: 256mb
os: Win xp pro with sp2
disk:40gb