Windows Messaging Question?

My coworker and I are attempting to make a cool program for the company (mainly because we're bored and are running out of projects) but we can't figure out how to send a message through Windows into another application.

For example, our program will message XnView (which is an image-viewer) and we can get it to increase and decrease the size, but we haven't figured out how to send 'PgDn' (which would go to the next image) to XnView.

I've scoured Google in the hopes that scan code, using either base10 or base16, will shed some light on the subject, but no go as of yet. Incidentally, '81' is the representation in scan code for 'PgDn', though we're not entirely sure how to send this in WM so that XnView will understand it.

Any help would be greatly appreciated!

prag