If you want to go from Windows 2000 to Windows XP you can perform a complete installation by loading the Windows XP installation while in 'Administrator' mode in Windows 2000.
This will allow you to do a fresh install or perform an upgrade of the existing Windows 2000 installation to Windows XP.
If you do not have a CDROM in your tablet PC, and a USB 2.0 or Firewire drive fails to work properly, your best case would be to buy a laptop-to-IDE connector and install Windows XP onto the drive while it's running on your desktop computer.
After the installation, you would have to change the 'IDE ATA/ATAPI controllers' from Intel/SiS/Whatever to 'Standard Dual Channel PCI IDE Controller' so that it will not blue screen as soon as you plug it back into the tablet.
Also, note that Tablet PC's have a specific version of Windows XP.