(Excel 2002, SP3; winXP, SP2)

This problem just started. When I open a particular .xls, my firewall
reports that Excel is trying to access the internet. This doesn't
happen with other Excel files. The IP address that it tries to access
belong to Savvis and Level Three Communications. The spreadsheet
doesn't have any (intentional) links of this sort.

From this, I have to wonder if this particular file has gotten
infected.

Is this even possible? The history of this xls file includes contact
with someone else's PC.

If a spreadsheet can be infected, is there a way to clean it? My AV
(AVG) is apparently not detecting anything.

Thanks,
George Eccles