There are a few different ways that Windows XP's new look can be changed.  Visit http://www.ThemeXP.org for the largest selection of Microsoft unsigned visual styles. 

NOTE: Before you will be able to use these styles, you will need to use a utility such as PatchXP or TGT Soft's Style XP to patch the skinning engine to allow you to use unsigned visual styles.

Update:  Read the Comments Below about Spyware! This can still be a good resource, but you need to be careful how you install the visual styles.