Minor "switch user" problem

Vista Home Premium 32, with all updates except SP1.

Say I'm logged in as my account, and leave the PC and it goes
into screen saver, which I have set to go back to he Log-in
screen.

Now my wife logs in with her account, does her work, and leaves
the PC, and it goes into screen saver mode, which again goes back
to the Log-in mode.

Now I go back to the PC, and click "Switch User" and I go the
screen with all the accounts, click on my account and re-enter p/w..
Sometimes, not all the time, when I do this, I momentarily get
logged in, then immediately goes into the "Locking Computer"
screen, and acts like it just went into screen saver mode, which
I then have to re-enter my p/w.

Any clues why Vista does this?

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- Zilla