Not to mention at least then you're SURE there is nothing left from an infestation. From a corporate standpoint, once the system is compromised, it's hard to trust, due to the commonality of rootkits, and VERY sneaky installation/survival mechanisms.

A system usually CAN be cleaned completely, but it WILL take some time, especially if it's heavily infested. It's often easier (and you can be more confident it's clean) if you just pull off your files (and scan them THOROUGHLY), and reload the OS. Not to mention, oftimes the infestation has damaged the OS as part of it's survival tricks, and the repair is harder/longer than a simple re-install.

I have seen it many times, a system appears to be clean, only to have it revert within days to a zombie.