Unless you're saying that the operating systems themselves are infected (I think someone would have noticed by this point), I always ended up reinstalling the system on the machines I purchased anyway, due the amount of bloatware many have on them. As a result, even if I were to suspect that, it wouldn't be a factor in the end.

But if you do find some sort of malware on a pre-assembled machine - I'm sure you'll have quite a few lawyers willing to take up the case for nothing, as you'll have quite a check waiting for you by the end of the proceedings!