System with 512Mb ram and 40 gb hard drive c drive is 1 gb free space and d drive 3 gb free space and virtual ram is set to 2 gb but still office 2007 excel is taking 99% CPU USAGE.aNTIVIRUS SEP11 UPDATED AND SCANNED NO VIRUS FOUND, COMBOFIX RUN NOTHING FOUND SUGGEST SOME TIPS.

From the limited specs you have posted I'd say this is an older machine, possibly a pentium 4 based model.

What you appear to be experiencing is bloat from Office 2007.
In all honest down grading to Office 2003 would make a lot of sense, as it isn't a graphically or system intense.