Basically it comes down to the difference between .Net 2.0 (2005) and .Net 3.5 (2008) Framework. Basically what you code in .Net 2.0 won't work the same if at all with .Net 3.5 framework. But fear not, with Visual Studio 2008 allows you to select .Net 2.0, .Net 3.0 or .Net 3.5 frame work to code in.
So feel free to start with .Net 2.0 (2005) but keep in mind, that this is older programming framework than 2008 so the code is not cross functionaly, but the principles are certainly a good building block.
Hope this helps