On the PS3, navigate the XMB to Settings. Then right at the bottom go into Network Settings. Once again at the bottom is Media Server Connection. Click this and Enable.
Now navigate back to the Video menu. You should now see "Search for Media Servers" and possibly the name of a computer/server on your network.
The next thing to do is load up Windows Media Player on the PC. Click the little arrow below Library and then Media Sharing (Library is moved a bit on Windows 7, but still visible from the main screen):
You will now get the screen shown below. If you get a screen before this just click allow and then this screen will show. Next tick the two boxes under Sharing settings. You may have noticed an Unknown Device show up in the white box. This is your PS3. Click on it and the press Allow. Finally press Apply.
If all has work you should now be able to go back to your PS3 and your computer's name will have appeared. Simply browse through this option on the PS3 to find all your media. Windows Media Player can now be closed and your PS3 will continue to stream the media, but don't turn off your computer.