Thank you for all! In fact I'm a system supporter, The computer is my customer's, Originally this computer use Kaspersky 6, and it infected with some Rootkit , Kaspersky ask for reboot to kill the virus , But when I reboot, Kaspersky 6 cannot kill, It always ask to reboot, So I think I must change a antivirus software, In China , It has already many people know AVAST! So I try the free version, The virus was so bad, I can get AVAST normally installed, But the computer was so slowly, It cost 2 hours for me to uninstall kaspersky 6 and install avast, It always pop up many ads windows, so I can not work normally. Finaly I get avast runs a dos boot kill, the system works OK! I think for the next time, I want to quickly get a dos boot kill (normally install the software), so I can work quickly, that's all ,Thanks all.