Now that you’ve got your computer ready, you’ll need to sniff out some websites offering HD content. Perhaps the best source is Apple’s Quicktime site which offers a whole selection of movie/documentary trailers and music videos, available in 480p, 720p and occasionally 1080p. Apple’s H.264 format downloads relatively quickly, although given the size of these files it is essential to have a broadband connection and ideally in the region of 2Meg or higher.
Microsoft, as ever, is keen to answer Apple’s challenge and it now has a large amount of HD content showcased on the Microsoft website. This is more about showing off the quality of WMV9 HD playback than simple trailers and so, whilst the clarity of the Windows Media option is undoubtedly greater, these files take a whole lot longer to download and are a bit more system intensive. However, if your machine is up to the task they are well worth a look.
Other places to check out HD content include TeamXbox’s high definition Xbox 360 game trailers here and DivX is also show casing its own HD material here. If you know of any other places to get HD content via the web we would love to hear from you via the comments forum; of course we realise that illegal file sharing networks are also a good source but as morally upright citizens we can’t condone that sort of thing.

There's a list of video clips at http://www.highdefforum.com/showthread.php?s=7b482c5ff0e86c4be54834bc7ba0576b&t=6537 which is comprehensive.
I find that Microsoft's WMV HD plays a lot better than Apple's H.264 files. I'm not sure about your comment that WMV-HD files are "a bit more system intensive" than H.264. Apple's first Quicktime release for Windows supporting H.264 certainly required an extremely high-performance system: see http://www.avforums.com/forums/showthread.php?t=218082&highlight=apple+quicktime.
As for needing a 2Mb connection, many people are not able to connect any faster than 512kpbs due to the distance to their exchange or the quality of their line. Others have a connection that is virtually useless at peak times due to congestion. Also, spare a thought for those without copper lines (ie. Aluminium or Fibre Optic) who cannot get ADSL at all.
It is perfectly possible to download a 70MB or so file on a connection slower than 2Mb, with some patience!
Cheers for the link. You are so right about slower broadband connections, it is down to paitence, though 8 meg connections via ADSL should be widely available by the summer. Some cable subscribers already have this as an option.