Writing rootkits is fun. I needed to a device driver for Windows XP that could hide a process, disable keyboard and mouse and hide directories, these are some definitions for a rootkit.
There are some inconveniences, like resetting the operating system because you did something wrong... things don't work like in userland.
Tools like Compuware VToolsD (for Windows 9x) and Compuware Driver Studio (for Windows NT+) make your life way easier. For the first you don't have to write code in assembler, you can write code in C and C++ and for the latter you can write code in C++.
I guess my next root kit, after the one for Windows 9x and the one for Windows XP, would be for Linux, but on Linux having access to the source code of the operating system is a big bonus.
In the end it's not that hard, with proper tools and with proper knowledge you can do anything :-)