I had an externnal user that was running Vista report this behavior.
He stated that another person at his company, also running Vista, was
able to open  and edit document  successfully. I installed Vista in a
virtual environment and was able to reproduce the problem.

Apparently there must be some settings in Vistas security
configuartion that need to be changed but because we don't support
Vista within our Enterprise I haven't taken time to see what they are.

Many things here to troubleshoot this issue. Ensure the Microsoft
Office version is up to date on the troubled workstation and is the same
version as on other workstations where the problem does not exist.
Review the SharePoint logs on the server located at c:\program
files\common files\microsoft shared\web server extensions\12\logs.
Check the event log  of the troubled machine.

I believe that would be a good start. Report your findings.

Yes, and further regarding Office versions, it can sometimes help to check the
version of the browser Office controls and their consistency between machines.
I've seen all kinds of weird  problems from mismatches between Office browser
controls and Office runtime libraries. Sometimes these can be fixed up manually
but often if this is the issue  you end up doing Office removals, repairs, and
reinstalls. That's the direction I find most productive most of the time.

I had the same problem recently and the way to solve it by modifying
the registery on that machine. Microsoft has a link you can go to that walks you
through the steps to do it. I have the link at work so I don't have it here. I
can send it to you tomorrow.

Is there any chance she has both Office 2003 and Office 2007 components
on that system? That's a known issue  and there's a patch for it.
There's a DLL conflict that keeps Office from working.