A few people have said that this has resolved the problem for them:

File|Properties|Summary Tab|Hyperlink Base
change it to C:\
(something that's always available)

I like to use the =hyperlink() worksheet function.  It has never failed me this
way:

=hyperlink("file:////yourpathtothe/something.xxx","Click me")

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If you have to change lots and lots and lots back...

You could use the code from David McRitchie's site:

http://www.mvps.org/dmcritchie/excel/buildtoc.htm
look for:
Fix Hyperlinks       (#FixHyperlinks)

If you're new to macros, you may want to read David McRitchie's intro at:
http://www.mvps.org/dmcritchie/excel/getstarted.htm

(If it's just a few, I'd do it manually.)