I would find necessary drivers you need first... and put them onto a USB key or burn them to a CD.
You may well have a CD that came with your computer which contains drivers, although they will be old now, but will probably still work find and let you use the PC adequately.
I advise people to locate drivers first, just in case XP doesn't find the drivers for your network card or motherboard chipset and you find you can't get online to download them once you've reformatted.
Of course if you've got another PC and a USB memory stick or CD burner then it's no problem. Windows will install and run, even if it can't find drivers for anything.
Take a few moments to double, triple and quadruple-check that you have backed up EVERYTHING you want to keep.
Did you put the CD in whilst Vista was running and select the "Install Windows XP" option? If so, that's probably why you didn't get the option to format the drive.
To (re)install XP, I always start the computer with the XP CD-ROM in the drive. This may require a change in your system BIOS setup. You will know if it's working if you get the message "Press any key to boot from CD ..."
Good luck, but I'd say find those drivers first. If you are struggling then post the make and model of your PC and we may be able to guide you in the right direction