The safest way is to backup all your personal files (music, videos, etc), you can backup to CD/DVD. The boot your computer from the Windows XP CD. When you get to the part where you select the hard disk you choose to delete the current partition (which probably uses your entire hard disk). Then select to create a new partition and enter a size. Not install Windows on that partition. You can add a second partition later in Windows, or you can do it at the same screen where you created the first partition.
If you have a Dell or some other retail computer your hard disk may contain a recovery partition and you may have a recovery CD that came with the system. Using those you can restore the computer to its original state. If you do not have a recovery partition and/or recovery CD you can't. All you can do it install a fresh copy of Windows.
If you have 2 partitions, use the second partition to store your personal files. If Windows crashes you can format the first partition without worrying about your data.
TheMajorMMX - Intel P200 MMX @ 225Mhz - V-Tech (PcPartner) Baby-AT mb w/ Intel Triton TX chipset - Nvidia Riva TNT2 M64 w/ 16MB - 256MB M-Tec SDRAM - Still running.....