The following will show you how to prevent spam without the use of those annoying captcha images. I use javascript, which according to W3C, approximately 95% of the people browsing the web have it enabled. The odds of one of your visitors having it disabled is slim. If it does concern you however, you could always place a notice to turn on javascript before using the form or you could always use another method, just place the other method in
tags.
Below is how you could prevent spam with simple javascript.
I have several sites where I use a pop up table to display information if the user rolls over and image.
Example, if the user wants to view notes on a certain client, they can roll over a little "note" image to view the notes about the client pulled from a SQL database.