This code will allow you to add a 5 seconds countdown to the download page of your site. Depending on how you code your download page, the following will simply add a label within your body tag as:
"Download will start in 5 seconds"
Controlled by JavaScript using the span tag.
This is similar to the site at sourceforge.net. When you download something on that site, you will be redirected to another page and give you a countdown before you receive the file.
Here's the full code for you to play with.