You will almost certainly have to go through the Windows Update loop several times. I reinstalled SWMBO'd Vista machine the other month as it threw a wobbly with some updates and refused to apply some. Windows Update support couldn't sort it out and it came down to a reinstall. Be aware that if the orginal disc/recovery partition is old the updates will pull down around a Gigabyte of data and it will take the best part of a day.
Even more annoyingly the first run will find 50 or 60 to apply, totaling sevral hundred MB, then on the second or third pass through the loop it will find a Vista Cumulative Update again several hundred MB. WTF doesn't it find that first! Oh and after that there is probably a "service pack" again several hundred MB...
Why paid for? Avast! works well enough here (free for home use). AVG Free seems to have got sensible again after it became slow and insisted on downloading all pages related to search results.