Vista - Windows vista "looses" activation

I have installed in a partition the Windows Vista Ultimate Retail, which was
activated some months ago without problem by telephone, since I don't have
intertet at home.

Since yesterday, almost all times I start Vista I get a message informing me
that the activation time has expired, and that I have to activate Vista.

I select again the activation by telephone and, as the numbers to be sent to
Microsoft has not changed (the hadware is the same), I reenter the numbers
that I received by telephone (which fortunately I had noted in a paper). The
Vista is activated and I can work with it without problems (even in the
explorer page of the computer it says that Vista is activated)

However, next time I restart the Vista, I get the same message, and the
process starts again...

Since I don't have Internet at home, the system has not been updated.

Is there any update to avoid this problem?? How can I solve it?