A pirated .iso already modifies the data from the official release because it has to break the activation code somehow. Thus, you can't check it against any official hash. From there, the best you can hope for is to find a "canonical" hacked version and check against that, but since most people aren't really capable of breaking protections, they'll trust whatever shows up on the pirate bay.
Most pirated ISOs I've seen aren't activation-cracked. I've had to download a few legitimately for systems that only came with a 'recovery partition', and whose hard drive failed or needed upgrading. I try to only download 'legit' ISOs (OEM or regular), or at least ISOs that look legit. It would be nice if I had some way of verifying them against each other to see if they've been modified.
Then it is your decision whether to buy a license or download an activator (which you can download after installing all the antivirus programs in the world)
