Technically speaking you can't have two Windows on the same PC as MS will only activate one license for it. However there is nothing to stop you from making a back up.
I would in you case clone the 36Gb disk into the new 250Gb disk. Remove the 36Gb disk and put away as the backup for safe keeping and starting using the new 250Gb disk right away. The difference between 250-36 = 214Gb is free hard disk space available for as many other OS installation as you wish to boot.
(1) How to clone the disk
Assuming your original disk is sda (in Linux term) for the 36Gb and the new 250Gb as sdb which you can verify in a boot-up Ubuntu by terminal command
The target sdb can be clone in about 10 to 15 minutes by terminal command in Ubuntu
Code:
dd if=/dev/sda of=/dev/sdb bs=32768
The only thing you have to be absolutely certain is the input file must be the 36Gb device which I have assumed to be sda, thinking Raptor should be a Sata and hooked to the first Sata port of your mobo. If you are unsure post the output of fdisk -l here.
Once you remove the old 36Gb from the PC you will find the new 250Gb hard disk boot exactly as the old one. XP will detect a change of the hard disk. This is allowed but it will demand an immediate reboot. After a reboot you can enjoy XP permanently as the legal copy already activated. You will find the back up copy is hardware-specific to your PC and cannot be used on any other machine. You can hook it back and run it as a 2nd copy but this is not advisable as XP makes a connection to MS site every time you are using the Internet and it will confuse them that the same IP address has XP changing repeatedly. The 36Gb may be formatted to work as a new disk after you are comfortable the migration has been 100% successful.
(2) How to install Ubuntu
Run the program "sudo cfdisk /dev/sda" in terminal mode of the boot up CD of Ubuntu. Again I am assuming the new disk is sda after you have removed the 36Gb disk forcing the old sdb to become sda. Use this program to create one logical partition of 1Gb large and Type 82 for swap and another logical partition of 10Gb and Type 83 for Ubuntu installation. Every partition you create in Linux will be Type 83 by default and you only need to change it to Tpe 82 for swap. You will find the two partitions will be called sda5 and sda6 respectively.
Click the hard disk installation icon on the boot-up screen of Ubuntu and install the Linux in the larger partition sda6. Basically you tell the installer sda6 is to be used for the mounting point of the "/" or the root of the Ubuntu. Have the boot loader Grub installed in the MBR of sda, the only hard disk in the system.
You should find upon a reboot after Ubuntu installation both XP and Linux will be available for your selection for booting. If this doesn't happen let us know. The cure is trivial.
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In Linux the installation and booting of the 2nd system is no different to the 100th system. Same method applies but you may need to configure the boot loader to minimize the work involved.