As far as hardware goes, there is no excuse these days for any even halfway modern computer not to be able to run a new title. Maybe they might be expected to cut the settings down but it should still run.
If a new title doesn't run on a relatively modern computer then then the programmers simply don't know how to(or didn't even bother to) optimize.
On the software end, Windows-Vista-only, in which companies go out of their way to make sure that the majority of PC owners can't use their title, is suicidal.