What will happen if I install .net 3.5?

Hi all,
I want to import a game I have, but apparently it requires net 3.5. Do you know if it will work or will it just crash?
What will happen if I install .net 3.5?
Microsoft is giving me the following error report:

setup.exe
Microsoft .NET Framework 3.5 SP1 has encountered a problem during setup. Setup
Any ideas?
Waiting to hear your views.
Thanks.
