Put in your Windows XP cd rom, Reboot your pc,If your PC reads from your cd rom drive first it should come up with an option of choosing run or boot from cd or hard drive. If your computer ignors this and just starts up as normal,this is because the bios is set to boot from your hard drive first missing out the cd rom. You will have to hold down the delete button while turning your computer on to get to the bios settings, In some cases it may be an alternative button like F6 key, sometimes it tells you at the start of booting your pc what key to press. Once in the Bios settings,you should have a menu with different things you can change, look for one that lets you put the boot sequence as cd rom first then hard drive second. save and exit once changed, now reboot with the xp cd in and it should ask if you want to boot from cd. press enter and then you can go through the process of a new instalation,make sure you do the format option to wipe the drive first so it can put a fresh copy of xp on your pc.You need to format it as xp wont go on a drive that already has a newer OS (vista) You will need your xp licence key at hand to type in during the process! hope this helps! I recomend xp or win 7,Dont need Vista, it may be newer than xp but it uses twice as much system memory while running than xp and win 7!