It is a huge challenge to enforce computer time limits because we (parents) have a conflict of interest - when kids are on the computer/TV we get quiet uninterrupted time in the house. I feel guilty when I let my kids stay on the computer for hours at a time. I built and now use a simple timer program that lets me set time limits (like 1 hour/weekday and no more than 30 minutes at one sitting). The program gives kids audible reminders like "5 minutes left" and logs them off when their time is up. It has ended all the fighting over computer time in our house. You can try it from http://www.TimesUpKidz.com
I try to steer my kids toward worthwhile computer activities, like browsing Google Earth (free) or drawing 3D buildings with Google Sketchup, also free. My younger son plays Viva Pinata, which is a great game; talk about non-violent - it's all about gardening! In the end, though, they just want to play games, which is fine with me as long as it is in moderation.