How do I go about if I want to execute a bang at startup?
I want to set the wallpaper in my personal.rc file.

One way to do that is with mzscript.
You have to have mzscript loaded and then use a step.rc entry such as:
*Script Start !wallpaper .stretch $Wallpaper$
The bang depend on whatever wallpaper mod you use of course.

go see http://loose-screws.com/modules.php
I'm not sure, but I'd guess that there's a module out there doing what you want (in case you don't want to use mzscript, which is probably the best solution anyhow... then why did I bother writing all this...?).

lswchanger.dll does it :)