Solver checked but doesn't appear

I have checked the Solver add-in in my Excel in Office XP and it goes
through the motions of installing it but Solver never appears in the
tools menu. It is in the Office10/Library/Solver directory.

When I double click on Solver.xla it tells me that it can't find some
guid in the registry. (When I do this at my work with Office 2000
Solver comes up in Excel.)

I have tried repairing my Office and removing and re-adding Solver but
nothing seems to work. I am afraid to reinstall Office because I really
need my Office right now.

I am using Windows XP Home with Service Pack 2.

I have seen this problem before in this newsgroup but I don't see any
way of fixing it.

It turns out I had to register the Microsoft dll FM20.dll that had
somehow been unregistered.