OK. These are the steps for the dual partition.

Connect the Laptop HD to the desktop. I would HIGHLY recommend removing your desktop HD, to keep from losing any files!

The following assumes you have a clean HD, no partitions. If you need help doing this PM me. (note: fdisk my test the HD between the steps below. Just let it finish.)

Boot from a boot floppy or disk (98 will work)

1. At the DOS prompt type "fdisk" (without the quotes)
2. You might be asked if you would like enable large disk support. If so, choose yes (Type "y" and press enter)
3. Select "Create DOS partition or Logical DOS Drive" (Type "1" and press enter)
4. Select "Create Primary DOS Partition" (Type "1" and press enter)
5. Enter "1500 and press enter (this is a gig and a half. if you go smaller, you may get an error in the setup. Better safe, then sorry!)
6. Fdisk will should to the main screen. If not press "esc"
You have just created the partition that dos and the xp setup files will stay in. Now lets do one for XP!
7. Select "Create DOS partition or Logical DOS Drive" (Type "1" and press enter)
8. Select "Create Extended DOS Partition" (Type "2" and press enter)
9. DOS should ask you if you would like to use the rest of the disk for this partition. Choose yes (Type "y" and press enter)
You have created the partitions, but for the second one to "show up" we must "assign a drive letter"
10. Fdisk should ask you something about a logical drive. leave the numbers alone and press enter. If DOS did not ask you, then follow steps 10a - 10c.
10a. Return to the main screen
10b. Select "Create DOS partition or Logical DOS Drive" (Type "1" and press enter)
10c. Select "Create Logical DOS Drive(s) in the Extended DOS Partition" (Type "3" and press enter)

OK. Now that's done, just reconnect your primary hard drive to your desktop, install dos, copy the xp setup files to the DOS Partition (the small one) and follow the steps below in windows. You must be in an administrator account.
To format NTFS:

1. Click start.
2. Click Control Panel
3. Click Administrative Tools
4. Double Click Computer Management
5. Click Disk Management on the left
6. Choose the appropriate disk (should be disk 2)
You should now see a graph on the bottom of the window
7. Right click the larger partition (should be on the right)
8. Click format
9. Make sure that it is in NTFS mode, and that "enable file and folder compression is not checked (it would be best not to do a quick format here)
10 Click ok

Yay now your done! Install XP like described in the tutorial!
Let me know if this helps!