unable to enter safe mode normally & winodows log out automatically, but not restart

this afternoon, when i was opening a web page, the avg pop-up and has detected an virus, and i checked back, most probably, the file name is " updates[1].cab" and the virus is "Win32/PEStealth"


my problem is that, my computer log out of window automatically without my autorization, and when i realized that, i tried to enter safe mode to have a scan on my computer, however, i found a problem in getting into safe mode.

on the black-background screen where i choose safe mode, i chose, but it move to another black-background screen and there is only one option " Window XP Home edition", so i chose it, the computer functioned well until the moment i chose the "User", it can't load a normal page of "Safe mode window" but all in BLACK too, with a box "cmd.exe" and i need to reboot the computer, and i have no problem in entering the normal Windows.

what can i do to remove it as my AVG can't scan any virus anymore.
i can't use CCLEANER as i fail to enter the safe mode.