The build time components of the WiX toolset require .NET Framework v3.5 or better. Windows XP SP2 or better should be sufficient as a build machine.
The install time components of the WiX toolset are written in native code and support Windows XP SP2 or better. No managed code is included automatically.
Install time custom actions may be written against DTF or custom BootstrapperApplications for Burn may use the Managed Bootstrapper Application framework. Those managed frameworks support the .NET Framework 2.0 or later.
Similarly, DTF may be used at run time and requires the .NET Framework 2.0 or later.