This tut will teach you how to stream music, photos, and videos (even MKV which the PS3 doesn't usually support). It's pretty simple too.
1: Go to this link and download the version that's best for you. I recommend the stable versions: Downloads - ps3mediaserver - Project Hosting on Google Code
2. Install the application
3. Open the application and turn on your PS3. If it doesn't already have the "Search Media Servers" option on your PS3 under Music, Videos, and Photos, go to Network Settings on the PS3 and turn it on
4. Search for Media Servers on your PS3 and eventually, the PS3 will pick up your computer. Go to the media server it found and browse through the folders until you find the location of the video, and that's it
Sometimes it may take a few tries and fooling around in order for the PS3 to find the media server
If the video/music you're playing is lagging, check out the FAQ section of the application