What is the make and model of the computer? On some computers, there are certain firmware drivers that need to be on the hard drive. On Dell for example, if you don't have the firmware drivers installed, the install might not work.
Do you have an XP pro SP3 disk?
Do you have a driver CD for the computer?
Are you going to save your personal files before you install XP?
Do you have the printer drivers CD for your printer?
Then you put the disk in the drive and start the computer, follow directions on the screen.