This is a pretty good 3rd party app that helps you modify the win7 shell with some ease - http://fav.me/d2lz8nw

Also, UxStyle also has some great methods through their forums and apps to help skin the whole UI or just the taskbar - http://uxstyle.com/

If you want to do it manually, I would suggest you use the reference guide provided in many win7 themes such as the Gaia09/10 downloads - http://fav.me/d29qufd - They are quite detailed on how to manually replace files in win7

Dont forget, you need to know is you are using the x86(32bit) or x64(64bit) when changing the files manually or you could do some serious damage to the system!