It can be setup as follows:

1. Go to http://www.google.com/webmasters/
(your site must be approved for access)
2. Go into “site configuration” under your website
3. Click Crawler Access
4. Click the “Generate robots.txt”
5. Under step 2, Specify additional rules, you can select the files you don’t want the spiders to crawl/index
6. create/download the generated robots.txt file
7. Upload the robots.txt file to your website.