According to the Netflix website, there is a limit to how many computer you can use to view Watch Instantly titles:
In addition, due to requirements from content providers, there's a limit to the number of unique devices each account can use to watch instantly over the course of a year. Each Netflix account may watch instantly on up to four unique PCs over the course of 12 months.
Tech-Recipes has a hack that might enable Watch Instantly on more than 4 computers:
This recipe will explain how to export the netflix registry key and then import it in to your registry on more than 4 different pcs after you have the netflix watch now viewer installed. I'll even offer a quick and easy solution to storing your keys and managing them
The registry key is: HKEY_CURRENT_USER\Software\Netflix\Movie Viewer\.
Thanks to Geoff and RP for sending this in.