UTStarcom PPC-6700 / XV-6700 / HTC Apache

I have owned a PPC-6700 by Sprint for about a month now. I'm still learning a lot about it but it seems like a very nice device, however, every application that I try to download from the internet (except from the Handango website) doesn't work. The error message is always the same: "This is not a valid pocket PC application".

What can I do about it?
Where can I download applications that will work with the PPC-6700 for free?

Thank you very much in advance

Enderson