It's not so much hard as tedious. You need to make sure you have all your drivers in advance. the general rundown is.
1. set the dvd/cd drive as the 1st boot device in bios.
2. Make sure only the drive you want the OS on is attached. Disconnect others.
3. Boot from the XP install cd and follow all prompts.
4. after installation, reconnect remaining drives.
5. Install all the drivers and windows updates.
Of course, if things don't go smoothly, it will require trouble shooting. See if you can find a youtube video. Some people do it slightly differently. I like to only have one drive in the system when installing an OS as it makes for less headaches down the road.
Both of the members above are more experienced than I , and are correct.
But be aware - you have to reinstall all of your programs. If you are missing the installation source, or the product key, you will have problems running the old software. You cannot just copy it from the old disk.
So if you have all your ducks in a row, it's a boring process that leaves you with a nice, squeaky-clean installation that runs faster than your old installation did.
Do backups every so often so that if something goes wrong you don't have to start from the very beginning.