Greg is right, an iframe is an easy way to do this. However, I've run into situations where the iframe will throw off session variables in IE, not sure if this impacts WordPress or not.
If you're going to create a page to house a WordPress install in an iframe, why not just have the link you would use to show the page with the iframe just link to a separate sub-domain where the WordPress install will reside?
My guess is you're not wanting to do a lot with theme development if you're wanting to throw WordPress into an iframe. If this is the case you have a few choices:
(a) google for a blank wordpress theme,
(b) develop a theme that looks like you're current site so that when a user clicks on a link, they won't know they're on a different platform,
(c) don't hide anything and make the WordPress install show up with a different theme. Consider American Express in their OPEN Forum site (http://www.openforum.com/), with their blog at http://blogs.openforum.com/ - same header, slightly different body and layout.
Issues w/ going the iframe route is that a WordPress site will grow in height, where you'll have to set the height of an iframe. You can control this by setting the height to something very large, but then your page will be very large, or you can control the amount of posts that show up in the WordPress admin.
My suggestions, scrap the iframe, install your WordPress on a sub-domain and then link to that sub domain instead of linking to your iframe page