Having your web application attempt to open client side programs poses obvious security risks, most browsers won't allow this.
You can probably implement some hacky IE only approach if you like, have a look here
http://stackoverflow.com/questions/776672/opening-outlook-through-javascript[
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If you simply want to display a new 'email screen', you can use the mailto approach.
If you actually want to automate outlook and perform various actions, I think the Activex approach is the only option - but it's really painful nowadays with all the increased security (good thing!) and should be avoided imo.