Know-How Exchange : E-Marketing

I do basis HTML editing myself on a relatively simple web site. I want to add simple ad serving - basically have a banner ad that appears on the page for x% of the page views (and other ad(s) for the rest of the percentage). Looking for some simple code that I can add into the HTML. I already use a "SCRIPT language=JavaScript" set of code in the page - perhaps something like this? Prefer not to add in MySQL or similar.

One thought that came to mind was code that looked at the system clock and calculated the tenth or hundredth of a second it is when the page was called, and used that to determine which banner ad to show. I remember using this back in college to make a random number generator for a computer class.

The end result once the random number is generated is that it will choose which set of "HREF=" to link to the advertiser's page and "IMG" code to show the ad.

Anyone know of some code?