migrating from .NET V2.0 to v3.5

Hello, I have Installed .NET Framework V2.0, I wish to migrate to V3.5. a) can I disregard the installed V2.0 and install upom it the V3.5. b) Provided I have an updated Windows XP Professional and IE V7, what additional prerequisite software are required in order to make it tick? c) Does the V3.5 has to be purchased? d) Being a novice to this site, can you please tell me where should I search for an answer, I mean on what URL, Much oblidged, Moshe