Write some software -- 2

I need you to develop some software for me. I would like this software to be developed for Windows using .[login to view URL], I have an very very simpleapplication developed by me in .net, it is working very fine, but not on windows XP. I want any .net professional to help me to make my application working on all operating systems. and other issue is, when I upload that on server and download it from there browser gives me alert that this file can harm your computer, and windows also gIves a security warning before installing. I am completely a newbie, don't know how to resolve these issue, but I am sure for any professional this is a work of not more than 20 mins. please reply...