I only use Vista through a virtual machine, and I've only played with it for
a few hours total. I also haven't used Office 97 in a number of years. But I
suspect the problem is related to permissions.
Office 97 installed add-ins in a subdirectory of C:\Program Files\Microsoft
Office (in Windows XP, at least). More recent versions of Office install
add-ins in a subdirectory of C:\Documents and
Settings\<username>\Application Data. In Vista and in XP, if the user does
not have admin privileges, then the user cannot change files within the
Program Files directory, but can change files within the Documents and
Settings directory (I'm using the XP directory names, so insert the
corresponding Vista names). By default, in XP, a user account has admin
privileges, so you would have no trouble running the add-in from Program
Files.
I wonder whether placing a copy of the Solver add-in somewhere in your
documents hierarchy, then using the Tools menu > Add-Ins > Browse button
would help you use Solver in your situation.
- Jon