How do I fix this problem uninstalling Microsoft Office 2010 (Beta)?

The very short answer to your question is this:

Restore to the full backup you took before installing Beta software.

If you didn't take a full backup (as you probably should have), then I'm not aware of a solution to the problem.

The other answer is that if you're actually part of the official Beta program for Microsoft Office then in addition to the Beta software you were also likely given access to Beta support - usually in the form of some private forums or other resources specifically for people in the Beta program.

If this is a common problem, then the others in the Beta program may have a solution for you.

Of course, as part of a formal Beta program, that solution is typically "Restore to the full backup you took before installing Beta software."

I understand that since you're asking me and not in some Beta program support, that you probably didn't take a full back up, and that none of this helps you. I'm actually sorry about that, but to be honest, it's the exact risk you were warned about when installing Beta software, and the exact risk you accepted by installing Beta software.

Beta software is software that's not finished. It has bugs - often many, sometimes crippling bugs. The Beta program is all about testing the software. Knowing that it has problems, and being clear that the program is not finished, a beta program enlists typical users to test the software and report issues that they encounter with it.

By using Beta software you are implicitly accepting this risk: that the software will have possibly serious bugs. In fact, the bugs it has may be so severe as to wipe out your hard disk.

Or simply not successfully uninstall.

In your shoes, I would investigate some of the official and unofficial forums that have appeared specifically for the Office 2010 Beta. The Microsoft forum is here. If you search for "Office 2010 beta Forum" you'll find several others as well.

If no answers are forthcoming, I'd consider trying something like Revo Uninstaller, followed by a re-install of your previous version of Office.

And when all else fails, a repair install of Windows, or a complete reformat and reinstall might be called for. (I'm honestly not sure if your situation warrants such drastic actions, but it's the one sure way to resolve the issue. And some problems with Beta software actually do result in this being the only solution.)

Unfortunately, the meaning of Beta has been diluted by Google keeping so many of it's live products labeled as Beta for so long. That's done the industry a grave disservice, in my opinion, as many people don't realize exactly what they're setting themselves up for when they install Beta software.

Needless to say, I strongly advise against it, in general, and then only of you're prepared for - however unlikely - potentially disastrous results. That preparation, as you might guess by now, includes a good backup that you can revert to.