Safe to uninstall .NET Framework 2.0 and 1.0 after installing 3.5?

You can remove the 1.1 Framework, but you cannot remove the 2.0 Framework

This is because the .NET 3.5 framework is an extension of the 2.0 framework, and not a separate one.

As well as the other (correct) answers stating that 3.5 depends on 2.0, I'd suggest that you don't remove 1.1 for compatibility reasons. MS did a pretty good job making 2.0 backwards-compatible, but it's not 100%. Any application written for and tested on 1.1 is more likely to work on that than on 2.0.