...did you try searching in Google before you made this? >> If you're using Windows 7 Starter, you can't change the desktop. But if you're using the regular 7, just do this. Click "Appearance and Personalization". And then a window will pop up that'll let you browse folders on your machine for images.
http://windows.microsoft.com/en-US/windows7/Personalize-your-computer Starter is designed to run on low-powered machines or netbooks, so it doesn't really allow for much customization in order for it to run as best as it can.
Thank goodness Microsoft had the brains to decide to remove that three applications running limit on Starter. I'm pretty sure adding the ability to change or remove the wallpaper would have no negative affect on performance. If you're able to do it in XP, you should be able to do it in Starter, or else I wouldn't even be an upgrade. A simple Google search for "windows 7 starter change background", let me find this. I haven't tested this myself, but if you really want to change your wallpaper, it looks like it will work.