you should boot to the windows xp cd and it will allow you to format.
Alternativly, it you want to repartition then goto
www.bootdisk.com and get
a win98 startup disk. Then goto
http://www.23cc.com/free-fdisk/ and get
thier free fdisk program. unzip it and copy the fdisk.ini and fdisk program
file in the /program foler of the file you just unziped: copy that to the
boot disk you just created and repartition form it. You can skip the second
part of this and just use the standard win 98 fdisk but some times it has
issues with large ntfs drives.
Alternativly if you have somethign like a hp or compaq computer there might
be a restore partiution in the existing hardriver. If you
delete/partition/format this, your not going to be able to restore without
getting new cds. Look at the computers manual to see if it has this feature.
most times you can press the F9 or F10 key to access it. It should start the
restore or install process and it will format for you durring the process.
just follow along and tell it to format.
Above all I would try start with looking inot the manufacturers handbook to
make sure your not going to do somethign they don't want you to. It is
possible they have a proceedure outlined under a section like restoring my
computer to factory settings or somethign simular. remeber when you format
you will loose all programs you installed as well as any documents oyu have
created. If you have the last 3 years of your taxes on it, it won't be there
after the format so get what you need of it first.