Save everything you need (documents, music, etc) onto a jump drive of some sort and pull all the aplicable drivers from your manufactruers website onto the jump drive as well. When done, remove the jump drive, physically swap the hard drives, power up and follow the instructions below.
1. Go into BIOS (usually press F8 when you turn on your computer)
2. Make sure the Boot Options are set to Boot to your CD/DVD drive first.
3. Insert XP CD and exit the BIOS saving changes.
4. When it reboots it will say something along the lines of "Press any key to boot to CD......"
5. Follow instructions and press any key
6. A blue screen (not the one you are probably used to seeing) will come up and load a bunch of stuff
7. Continue and follow all of the steps
8. Make sure you elect to NOT recover and continue to install
9. You will see a list of all partitions on the physical drive. Delete them ALL and confirm that yes you know you will lose everything.
10. You should be looking at the only option that says "unpartioned space" select it. (saying to install windows over the entire hard drive)
11. This is the reformat part. Select the non-quick one (because it's the first time you are using the drive.
12. Go make some coffee or watch TV. Let it do it's work and reboot on it's own (CD still in), expect 30-90 minutes here.
13. Eventually it will start asking you questions, fill all out appropriately and you're done.