The SQL Server Premier Field Engineer blog has a post about an issue with installing SQL Server 2008 on a system where the .NET Framework 4.0 is already installed:
The first workaround is probably the easiest for most folks, which is to copy the install files locally and then perform the install. This isn't a SQL Server problem, per se, as the blog post indicates. Rather, it's due to the tightening down of some security in .NET.