Windows contains a version of .NET by default. Here's a listing of them.
XP
- .NET v1.0 -- Service pack 1
- .NET v2.0 -- Service packs 2 & 3
Vista
- .NET v3.0 -- All service packs
Windows 7
- .NET v3.5 -- All versions and service packs
Windows 8
- .NET v4.0 + Metro UI available.
Windows 8.1
- .Net v4.5 + Metro UI available.
Windows 10
- .Net v4.6 + Metro UI + Universal Apps available.