Recently I have had more and more requests to help people add a signature in Outlook Express. I figured now is as good of a time as any to make this post! I'll be showing screenshots from Outlook Express in Windows XP, but the same steps (or fairly similar) will apply in Outlook, as well as Windows Mail.
Here are the steps to add a signature to your email:

When your email is first opened, click on tools and then options:



In Options click the 'signature' tab:



In the 'signature' tab, click on new:



From here you need to make a decision: do you want your signature to be a text-only signature, or would you like to have a graphical email signature? If you would like to use a text-only signature, fill out the 'text' box as seen here:



If you prefer to have your picture or some type of color/font formatting, choose file. The one thing you need if you choose file is just that- a file. If you are using Outlook or have Microsoft Office, you are in luck! Microsoft Word can create html documents complete with picture, color, and different fonts! Open Word and create the desired signature (you can drag and drop pictures in the file if you want them). Save the file as an .html file, and make sure to note the location of the saved file. From here just choose 'browse' to the right of the file field, and select your file (you may need to change the 'files of type' field to html). Click on open to set your signature to your newly created html document!

The final step to make sure that your signature is included in your email is to check the box labeled "Include signatures in all outgoing messages":



From here click on OK and you are all set with your own customized email signature complete with picture and formatting!

I am totally aware that there are some minor differences between the different email programs. If you need specific help with a different email program (Outlook 2007, Windows Live Mail, Mac Mail, etc), please ask!