if yrags' idea doesn't help, try this:
start/run - type "services.msc" (no quotes), right-click on "NVIDIA display driver services" and select properties. In the 'startup type' box, select "disabled. Then restart your PC. If it doesn't fix the WB problem, you can always turn it back on (just follow the same procedures, except select Automatic instead of disabled).
I disabled the service, rebooted the computer, ran services.msc again to confirm that the service was not running and changed my WB skin. The problem remains - the skin is applied just fine, then right at the end the winlogon.exe exception pops up.