You're right. Try giving your Windows user account full rights on the working copy folder. It worked for me too. From what I experienced, this is a kind of logical issue. Actually it is not even an issue, but a normal thing. In brief words: when one Windows user works on a working copy, he locks the working copy for himself. If then another user tries to work on that same working copy, he cannot, because the previous user still works on it, thus it is still locked by the first user (until he commits and releases the lock).
So yes. In case you have no other option, give your Windows user account full rights on the folder if you can.
My case was the following. I have a portable hard drive on which I have the working copy. Yesterday I worked with that hard drive on one computer and I did not commit. Today I had to use a different computer, thus under a different Windows user but with that same portable hard drive. Of course the working copy was locked and I could not update, nor commit, nor clean... I could do nothing. So I gave my current user full rights on that folder and it worked.
Hope it makes sense now. :)
commit
. – Scintillate