WinXP strange logoff/shutdown screen

I have a problem. A client of mine has a windows xp machine that has a strange logoff and shutdown screen. In the left upper corner of shutdown screen appear the letters ” L O W “. The same in logoff screen where an ” A ” inside a ” T ” also appearss. Its not a Windows XP theme, nor a virus. I have checked the machine with NAV 2004 ( with latest definitions ), with AVG antivirus ( latest definitions ), Spybot S&D, Ad-Aware, and some other programs.all appear to be normal.