Windows updates changes my password??? any ideas why???

This may be easy for someone to sort out for me. i have Windows XP
Professional. I have a password that windows asks me for when I switch the
computer on, fine then I go to my usual desktop, then while I'm online a
windows update is downloaded and then it asks me to restart my computerto
install the update, so far so good, after the computer has restarted, now
this is where the problem is, it doesn't recognise the password, si I have to
restart the computer and press F8 to go back to last known good
configuration, where the compuetr recognises my password and everything is
fine again........until I get an update and go through the same commotion
again, and as I have to always go back to last known good configuration my
computer never gets any updates installed. Any idea why it doesn't recognise
my password after an update is installed, its driving me BARMY. Thanks