Yes it's actually really simple.
First on your PC, go to Windows Media Player 11. Then right click on the "Library" tab. Go to "Media Sharing" and tick "Share my media to:".
Make sure your PS3 is turned on, and it should appear as "Unknown Device" in the dialogue. Click its icon and then click "Allow" and Ok - Your PC is now a media server.
On your PS3, your computer will appear with a white icon under the tabs Music Picture and Video (you will need to press Find Media Servers first - also make sure the option is on in your Network Settings).