and how can i make my computer like the way it came from dell?
The computer shipped with a system restore CD. Pop it in, boot up the computer and follow any prompts you get.
You can easily view the partitions on a drive by: right-click "my computer" (or whatever you may have renamed it to, click Manage, then single-click Disk Management on the left pane. Then, on the right pane you will see all drives, even the ones that don't have a "letter" associated with them, and their partitions and partition type, along with the file system being used in each partition.
It won't let you delete the partition that your system or swapfile is on, but any inactive partitions can be deleted there. Chances are, you only have 2 partitions. One is a VERY small hidden restore partition from Dell (that won't show up there), which used to be associated with their "zztop" utility, and the other is the mass system partition.
If you have PartitionMagic (which I'm sure you don't), you can manipulate the location and size of any/all partitions on your system (to an extent).
If you do not have your Dell recovery CD, but you do have a WinXP CD, you can pop that in. When prompted "hit any key to boot from CD", hit a key. Select "Setup Windows XP" (not "repair...") and when it says "there is an existing installation of XP, what do you want to do?: Repair, install new anyway..." select Repair. That is basically installing XP over the top but completely re-"finding" and installing all of your hardware. This is also a process you can do when you upgrade or change your computer, so that you don't have to reinstall everything again. Be sure you have your CD key because it will ask you for it again.
If you want to wipe it out and start fresh, boot from CD, select "setup Windows" then when it finds your existing partition, click Delete, Enter, L, and then select "use this partition". You can use "quick format" if you want to save a bit of time. make sure your file system type is NTFS from the list.
Boom, that's how ya' do it.