You cannot "disable" .NET -- the CLR is a foundation component of Windows itself. You could uninstall it on Windows XP (not on later versions though, and only because Windows XP doesn't come with .NET in the first place), but that's the only solution. And many applications will simply cease to function without the runtime installed.
It should be noted that
aspnet_regiis.exe -u does not disable the .NET runtime. It turns off ASP.NET support in Internet Information Services (which you should not be running on Windows XP anyway). .NET applications will still run even after that switch is applied.
Finally, I must point out that .NET is just like Flash or Java -- it's a foundation library required for many useful applications to work. There's really no reason to turn it off, any more than there is reason to ban all checkbox controls.