Excel Hyperlink Issue

I have a client that is having problems with a few excel spreadsheets that they use for order tracking. In these documents they have several hyperlinks that point to PDF documents. Both the excel files and the PDF documents reside on a 2003 Standard server. Most of the users are running Windows 7 with office 2010.  Some however are XP using Office 2007.  There are usually a few users in the file at the same time, but only one person has read/write access at a time. The other see it in read only mode.

Randomly Excel will change their hyperlinks so that instead of when clicking on the link it opens the specific pdf, it will change all of them so when you click on a link it just opens the folder in which the pdf documents are. This is an improvement over what it was doing before  of just forgetting what was there all together. I got it to at least go to the folder after I setup the hyperlink base in all the documents to start in the folder where all the pdf docs are.  

At this point however they are getting really tired of having to remap all the hyperlinks every time they change. Does anyone know of a way to get this to stop happening?