C# VS 2010 older versions of .NET

I've personally never worked with backwards compatibility, pretty much everyone uses the latest stable version (3.5 right now, but not for much longer). Since the .net framework is so high level, there isn't a ton you can do to make your code backwards compatible. You are pretty much at the mercy of the compiler. You can have different version apps on one computer as long as they are all installed though. It may work different for asp.net.

I would advise learning 3.5 right now then learn the differences in 4. The two are not very different, most of the changes are additions or advanced modifications.