Booting to DOS

I'm not sure what you mean by runninx XP from DOS. To run XP you have to install it, and to do that you must boot from the installation CD (or from a DOS-CD and run the XP *installer* from there)

If you can't boot from the CD you might try to see the BIOS settings and check the boot sequence; it might be disabled.

If you format the harddrive Vista will be gone, but your machine won't start without a boot CD until you install XP (or a different Operating System) on it.

To get into BIOS, watch for what key to press when your machine starts. Usually says "Press F1 to enter BIOS" or something like that. Just press and hold that key until the BIOS settings appear.

BE careful, though: messing around here could cayse a lot of problems if you don't know what you are doing. The "Exit without saving" option is the way out if you are unsure.

You could try bootinge from a CD. You can get some really good ones here: http://www.ultimatebootcd.com/

If you have XP installation CDs you should be able to boot frmo them and go along from there.