.Net 2.0, 3.x (avoid using that version btw) and 4.x can happily coexist on a single system. When installing 4.5 on a system where 4.0 already exists, it will be upgraded to 4.5. (Win Vista and later, as said above)
.Net 4.5 is fully backward compatible down to 2.0, except for the new/improved features and, hopefully, for all the bugs in these previous versions