Thank you whocares.
The OS is Windows XP with FAT32, the virus is some kind of boot virus(detected by some other AV released more than 1 year ago, so the boot virus can’t be cleaned). the mouse froze at the center of the desktop windows just after the Windows started up and desktop appeared. then I could do nothing.
I am also very new to avast!4 free edtion and don’t know how to use it.