Forget password for xp pro sp3 standalone desktop.

I have a office pc working on windows xp professional (not connected to internet) and recently upgraded to sp3. When I am working the kasporsky antivirus show me some cautions about virus and upgrades and click OK, instructed by it. After that it processed some time and restarts and shows me the "log on to windows xp professional" screen asking me for username and password. I know the user name but forget the password. Several times I attempted restart with safe mode and typed " Administrator" in the username box and enter. But the same screen reappers. I even tried the Kasporsky bootable rescue disk 10 also. It scans and clears the virus, but my problem still persists. Kindly help me to open my pc please.