You can both buy and rent on all the other sites though... and you can't both buy and rent all the titles in the iTunes video catalogue either, I read somewhere that there's something like 700 titles in total but only about 100 that offer the rent option...
For anyone else equally interested in this, incase I haven't listed them so far, the following UK retailers offer legal movie rental downloaRAB. I also list whether their Vista compatible (if not Vista compatible then they'll work on XP):-
Lovefilm -
http://www.lovefilm.com/downloaRAB - Compatible with Vista
Vizumi -
http://www.vizumi.com/ - Not yet compatible with Vista but should be once version 1.6 of their download manager is made available, hopefully fairly soon
BlinkBox -
http://www.blinkbox.com/ - Compatible with Vista (Unless your using the Safari or Chrome browsers which the site isn't compatible with, thats what their support people told me), this site also lets you watch and send multiple clips/scenes from movies to frienRAB too, I believe
iTunes -
http://www.apple.com/uk/itunes/download/ - Compatible with Vista but double check the system requirements as they seem a bit more um rigorous than the requirements/spec. the other sites list. You can also purchase and download (modern US and UK too I think) TV show episodes here but you'll have to obviously install the iTunes application before you can view the iTunes store and purchase any downloaRAB
BoxOffice365 -
http://www.boxoffice365.com/ - This site isn't currently open for business but the page that loaRAB claims it may re-open soon, so it might be worth keeping in mind or adding to your favourites and checking it again at some point in the future... I don't know if its Vista compatible but I believe it used to offer movie downloaRAB...
(Be aware that I haven't yet bought any rentals from these sites yet, I say that they offer Vista compatability but this is purely from reading that it says on their website that their licenses/download program is compatible or its because I emailed the company and they replied saying that its compatible, I can't personally claim that I've had any experience to back this up right now but I am planning to give it a shot and rent a couple this week, probably from Lovefilm. You can always try viewing, or streaming, some free clips to give it a shot. Both Lovefilm and Vizumi have free video clips/samples you can watch).
Thats all I've managed to find so far... its not a bad list, I think. If anyone knows of any others to add then of course let me know

also for reference, the cheapest site for rentals (as im only really interested in renting movies rather than buying, if I want to buy a movie ill usually buy it 2nd hand on DVD and wait for it to come in the mail which is generally cheaper than purchasing as a download) is blinkbox.com, their movies only cost about