I just built a new computer and I am trying to keep it as "clean" as possible. I have the purchaised version of VisualStudio 2008 but I dont know wether I should install that, or the latest express version?
Since you're trying to keep things "clean" (whatever that's supposed to mean here), I'm guessing installing both is unacceptable. I'd suggest that unless you're using some feature in the 2008 version that isn't available in the free Express version, go with the more recent of the two.
You can install both if you want to as long as your computer has sufficient free disk space. If I were you I'd get Visual Studio 2012 Express and use VC++ 2008 only if you need to. Microsoft put all their compuiler into a single package, you can't just get VC++ 2012.
I have both 2012 Express and 2012 Pro installed on the same computer with no problems. I was using the Express version for awhile until I came upon something that it could not do. So, since I hade not purchased a Pro version since VC++ 6.0 I decided to just buy it. I could now uninstall the Express version because it is no longer of any practical use.