Since Visual Studio 2010 will convert and open Visual Studio 2008 solutions technically you don't need to have 2008. But if you have to have for some reason such as sharing between users on a source control system, it's recommended that you install the oldest version of Visual Studio first (in your case 2008) and move up to the newest version. As you install each version, it's also recommended that you run any service packs and/or fixes before installing the next version in line.
As an aside, generally when opening a 2008 solution in 2010 for the first time you will be presented with a dialog letting you know that the prior project is set to run on a older runtime and will ask if you would like to upgrade. Upgrading projects is generally not a breaking change like it was when you moved from .net 1.1 to a .net 2.0 project and in the end will give you a newer environment to work in along with newer framework options to work with.
So, uninstall everything. Clean the machine of all registry errors and then install from oldest to newest.
Good luck and hope this helps some.