How to format hard disk and change the os from win 7 to xp

do you have a copy of windows xp? You first need to create a new drive partition in windows 7. you right click on "computer" in your start menu, and select "manage". then you need to go to disk management. shrink your main partition, which should be your largest, unless you have multiple os's already. You right click the partition to select shrink, then it will give you a set of dialogue boxes asking how much you want to shrink it by. it will then become unallocated space. You need to right click it and select new partition. You would probably want to use all of that space for your new partition, and format. then you just put your windows xp cd in your cd drive, and restart your computer. while it boots, you need to go into your motherboard settings. You then select the option to boot from disk, and then exit those settings. It will boot into the windows xp installer after that. You just need to do a custom install, so that it installs on your new partition and not overwriting your windows 7 partition.

if you don't have a copy of windows xp, you're not able to do it.

this should probably be moved to either the windows xp or windows 7 forums.

The first question is WHY? If it is for some older programs, there are work arounds. such as using compatability mode when installing, or a virual box to run them in.

Any way, remocmend method.
(1) create a windows system retore back up - makes it easy to revert back to windows 7.
... Under control panel select create a image of your System drive (C Drive). You can put his image on a 2nd HDD, a Usb HDD, or on DVDs.
(2) Back up all of your data as you will wipe the drive.
(3) go to MB manuf (or laptop manuf) support website and down load ALL need Drivers. Windows XP has very poor native support compared to Windows 7
(4) Insert XP installation disk and select custom install, DELETE all partitions. NOTE if it will not let you, then insert a Win 7 disk and again select custom install and Delete ALL partitions. Exit win 7 installation and re-insert Win XP disk.
(5) Complete win XP installation.

ADDED: based on above post.
If you want to daul boot win 7 and XP - I do not recommend that method. Reason - Requires a software boot manager that if a problem develops can prevent booting to either system.
(1) Buy a small HDD (ie 320 Gig -> 500 gig) only about $50.
(2) disconnect Win 7 HDD, and connect up the New HDD to a sata Port, Other than the one the Win 7 HDD was On.
(3) Download ALL divers as Outlined above)
(4) install win XP, and drivers.
(5) when done, re attach win 7 HDD.
(6) use the Hot Key to bring up a boot menu (ie F12 on my gigabyte MB, F11 on my Asrock MB). and select which HDD to boot to.
NOTE: set Bios Boot priority to the HDD with the most often used Operating System on it. If no Hot key, then when you want the other operating system, then just change the Boot priority in bios.