Why don't you also check to make sure they're not running a pirated version of Photoshop? Or Half-Life? How about Microsoft Office?
See where I'm going with this? I don't particularly agree with piracy checks as it stands - but in order to perhaps save you a headache, I would suggest that you only worry about whether your own software is pirated, using whatever means you wish (licence keys, phoning home, whatever nefarious and intrusive method you so desire!). So someone who pirated Windows MAY be likely to also pirate your software too. If you intend to block usage of your software if you detect they're running a pirated copy of Windows - well, that's just bad practice. What if the validation software is buggy, or incorrect, or reports that the software is pirated because SOMEBODY ELSE used this person's legitimately bought licence key?
Aside from it being none of your business, Windows in particular has been known to falsely report that a legitimate copy of Windows was pirated. You should not trust anybody's validation tools but your own (and even then you might have coded a crappy validation tool!). There are so many things that could cause an incorrect piracy report - I would steer way clear of this approach.
