This virus has 2 versions:
Shaker.373 is a relatively harmless memory resident parasitic infector. It copies itself into the interrupt vector table, and hooks INT 1Ch and 21h. Then it infects COM or EXE files that are executed. Sometimes it 'shakes' the screen.
Shaker.409 is a benign memory resident parasitic infector. It hooks INT 21h, and infects COM files that are executed. On Fridays, it also hooks INT 1Ch, and from 11:00 until 11:20, 'shakes' the screen.