Author's Note: The bad guys win in this one. Warning for murder of innocents.
Harry Potter's address was on file with the Ministey, and not hard to discreetly retrieve for a man with connections. Dumbledore, of course, was not a total fool: the first man who approached the filthy Muggle house had his Marked arm explode into flames. Since the boy lived at his filthy Mudblood mother's even filthier Muggle sister's residence, it was likely that Dumbledore had set up some sort of blood-based protection.
Thus the wizard who drew the short straw was set on full-time monitoring duty of the Muggle family - a demeaning task if there ever was one. It only took once, however...
And his labor was rewarded after several months when the Muggles left their residence on a family holiday. A group waylaid their vehicle on the way to their destination, and that was the end of them. Fun as it would have been to sport with the Muggles as in the old days, action had to be taken quickly before Dumbledore intervened. As such, the moment the Muggles' throats were cut, the man left behind on Privet Drive blew down the door of the neighboring house and dispatched both Squib and boy with a single spell.
That night, the Dark Mark lit up the sky for the first time in seven years, and it was beautiful in their eyes.
Author's Note: We know Harry's location is obtainable by non-Ministry officials because Lucius sent Dobby there in second year. We also know that, for all that Harry is forced to live at Privet Drive, the Dursleys weren't subject to the same restriction - they occasionally took off and left him with Mrs. Figg. And there's no blood protection on them.