lets say I have managed to hack over 100 PCs (not powerful , all in the same building)
what kind of things should I do to them ?
*mining bitcoin is out - those computers are really bad*

Um DOS Botnet? Like there's not a lot of legal stuff you can do with 100 hacked machines, like Bitcoin or altcoin mining would be the only things that come to mind

Use them as load balancers for your web services

Make them click on your ads.

Step 1: Document step by step how you someone would be able to accomplish this exploitation, potential impacts, suggest corrective actions, and supply said documentation to the affected party(ies).

Step 2: Feel good about using your powers for good and not evil.

Step 3: Profit.

let them do something good: http://www.worldcommunitygrid.org/

But really you should report the security problem to the people responsible.