It has a limit of 3 installs, so it may not even work. But it was $1.65. And I have Webroot now, so I know the software itself works pretty well.
Throw it in a pc you dont care about and see what it does.
It probably wont work. When I built my computer recently I was cleaning out some old components and came across my old webroot CD. I through it in and they said the CD key was obsolete.
CPU
CPU Cooler
Motherboard
Memory
Storage
Video Card
Power Supply
Case