This hacking support software identifies idle resources on the host sytem and briefly commandeers them to accelerate the processing of the hacker's intrusion efforts.
Overclock Software is used to significantly increase the speed of capture and fortify operations for a few seconds, giving the player a huge edge over the diagnostic subroutine, which one can further exploit by using Stop! software to paralyze the besieged system.
It is by far the rarest hacking software, with most copies part of the loot of high-level hacks, or purchased from merchants.
The item is sold by the merchants Costache in Prague and Entity in Golem City, and can occasionally be found if you do some exploration. Of note, the shelf in Adam's bedroom has a copy of overclock software free for the taking.