LisaK, we have this same issue in our Excel 2007 files sometimes. I am
really curious if you have solved it.

We have hyperlinks to files on our internal LAN SharePoint servers. The
hyperlinks work fine for me and others. But every so often someone opens the
Excel file and resaves it (without deliberately editting any of the
hyperlinks) and suddenly all of the hyperlinks no longer point to the server
location but instead to that person's C:\ drive (which of course is wrong, so
the link is broken now forever). Re-fixed the hyperlink just starts the
process all over again, the links always break/re-point to the wrong
location.

Folks are not saving the file to their C: drive at all. It is very
strange, and I would like someone from Microsoft (or others who can) to
explain why Excel would change the "absolute" (not "relative") file path
designated in the Hyperlink in Excel 2007.