<p>I'm thinking this might be a quick and easy way to lower the form spam on our site just a little bit. The idea being that (I have read) spammers aren't running with javascript enabled. (Or at least they are accessing your website without running javascript. I.e., they aren't browsing up to it in IE or FF.</p>

<p>I can use .asp or .aspx.</p>

The simplest way is to set a cookie via javascript and check for it on postback.However, if you're looking to minimize spam you should actually have the browser perform a simple task which requires javascript execution. See Phil Haack's "Invisibile Captcha Validator" control, which has since been included in his Subkismet library:
http://haacked.com/archive/2006/09/26/Lightweight_Invisible_CAPTCHA_Validator_Control.aspx