What do clients need to run .NET applications?

I want to start in on .NET, but my clients are pretty behind the times, and so I'm stuck with VB6, I guess. Upgrading is not an option, as it's a government job, and they don't know what upgrading is. Minimal upgrades on a one-by-one basis are acceptable, but I want some more info before I go waste my time with .NET.

?