I believe I have my answer but I just want to be sure. Basically I repair computers, most of unknown origins. Of course some could have viruses and such so I was wondering what the most efficient way of achieving some sort of isolation would be. I seen someone recommend having a second router. This interested me because I do actually have 2 routers one running on 192.168.1.x (the main router) then a private router in the back of my house which I use running on 192.168.2.x I have noticed that I can't talk to the devices on router 1 and vice versa. Will setting up a 3rd router on 192.168.3.x provide sufficient isolation (of course I would only connect the test machines to this). Currently I try to avoid any sort of network access on most machines however in some cases (repairing others PCs) this may not be an option. If this isn't a sufficient solution then can you please provide me with one and preferably one that won't kill me (cost wise) assuming the recommendation includes purchasing hardware firewalls for a DMZ or something alike.
On a side note, I'm aware of some viruses which like to stick around even after a simple formatting. Will a zero fill tool like autoclave get rid of those? Is there another method to ensure that an HDD is clean of all viruses?