Microsoft is introducing a new version of its online video site, MSN Video.

The big change is the ad model. Microsoft will begin to show ads to users based on time spent on the site, rather than just showing an ad for every clip a user watches.

Users of MSN Video will now see a pre-roll ad on the first video they watch, and will then be shown subsquent ads at intervals, regardless of how many different clips they view.

From a user’s perspective, avoiding pre-roll ads for every clip you view may be a good thing, but if ads are shown too frequently (three minutes has been touted), or in the middle of a music clip you are watching, this could prove to be even more annoying than watching a pre-roll on every clip.

Google has been more careful not to annoy YouTube’s users, and has tested a number of ad formats before opting for animated overlays which appear in the bottom 20% of the screen, rather then pre-rolls.

Other changes to MSN Video include the ability to search across MSN Video and user-generated channel Soapbox, as well as video content from 40 different providers on the rest of the web.

The new version of MSN Video is available in the US, with beta versions launched  in Germany, Italy, Spain and the U.K.

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