The size of a program changes. The disk runs out of free space when it seems to have plenty. Bit errors occur frequently while running Windows. The disk drive is active when it should not be. The hard drive is inaccessible when booting from a floppy drive.
Unrecognized files appear.
File names change. The keyboard makes a clicking noise. The screen becomes distorted. Text on screen does unusual things. After a virus is detected, it is important to isolate and eliminate it as soon as possible.