Excel 2003 has Information Rights Management (IRM), but I have not read much
feedback on this. I believe it requires a company to have a Rights
Management Server. It is under <File> <Permission> <Restrict Permission
As..>
<...copy>
Users or groups can be given a set of permissions according to the access
levels assigned to them by authors using the Permission dialog box:
Users with Read access can read a document, workbook, or presentation, but
they don't have permission to edit, print, or copy.
<...end>
Internet Explorer has an add-on:
Rights Management Add-on for Internet Explorer
http://www.microsoft.com/windows/ie/downloads/addon/default.asp