First you need this really useful plugin from telegraphics.com.au It works fine with Photoshop CS3 for PC.

• Create a new Photoshop document 16x16 pixels or a multiple of this such as 32x32, 64x64.  We can optimise the image size later on.
• If you want a transparent favicon, make sure the background of the image is transparent.  If not, a quick way of removing the background is to select it using the magic wand and hitting delete.  or select the background with the magic wand - Click the Photoshop Select menu and select Similar. That should select all the background in one hit - hit the delete key to remove the background.
• Save this as your Photoshop favicon Master
• Now go to File -  Save for Web & Devices in the Photoshop menu
• Select GIF, click transparency on if required (diffusion transparency filter should be fine), set image size to 16x16 pixels and save.
• Reopen this gif file in Photoshop and finally save it as an icon file.