XP Pro with SP3 sign in question

When my pc goes into sleep mode and left overnight, on awakening the pc, I
sign in with user name and password, pc goes through its normal startup, but
then goes back to the sign in screen again. I insert password and user name
again, and it then restarts as expected. Any reason why this happens,
signing in twice ?
Only one other user on the pc, who has a separate sign in and password.

Machine runs slow, IE can take several minutes to fire up.
All the usual done, defrag, scandisk, virus/malware checks etc, including
regedit.
Bob