Archived from groups: microsoft.public.windowsxp.security_admin (More info?)

I have Win XP Professional w/ SP2 installed. My PC is losing login account  
password if I keep it on for several hours without using(Kept it ON for 6  
hours to update windows and second time 4 hours to scan virus). It shows an  
error message saying that I can not login because characters length is  
incorrect. Btw, this message only appears if I type correct password,incase  
of wrong password its shows regular "forgot ur password?" ballon.
I have Norton 2005 and microsoft anti-spyware installed.  XP, Norton and  
spyware are up-to-date...  
Thanks for your help in advance