Is it easy to install windows on a second hard drive?
Yes. If you use XP or Vista it will install a boot manager that will give you the option of which operating system you wish to use.
To do this. Set up hard drive in machine. Load the computer to desktop and insert the windows cd. Pick the drive you want windows on. It will check the drive and reboot. Just follow the instructions and make sure you pick the correct drive for you new install.
When done, you will have the choice, before windows loads, of which hard drive you want to boot from.
You can do it the same with a caddy fitted.