boot up irregularities

I have a WinXP PC. In the past two days my computer has been acting strange.
I have three accounts setup on my pc. One is admin and the other two are
regular usernames. Yesterday I got a message that my computer was locked
out and only the admin could access it. However, it showed a field for a
username and password but it would not allow anything to be typed in. The
only way I was able to get rid of the pop up box was by turning off the
power switch.
Today I booted my computer and the computer went into the CMOS setup. I dont
know if there was any messages before that as I booted up and left a minute
so it could finish booting. I selected exit without saving and everything
seems to be working fine.

Do I have some kind of trojan or virus? My Norton AV didnt show anything.

Ronald Smyth