My options, xp wont load after hardware upgrades

I have a built desktop that might be about 5 yrs old. it was running xp sp2. I thought I could do some hardware upgrades.... New mobo, i7 cpu, graphics, added sata drive in addition to my 2 ide drives. Now xp wont even get through safe mode at all without just rebooting.

I understand now from a little reading that I need to repair xp with a an xp disk. The problem is that the boot drive I am using with the xp operating system is my Mom's old computer that I inherited recently and I can find a xp install disk anywhere.

What are my options here. I don't think I can walk in to the stores and buy xp anymore right? Do I have to upgrade to Vista? Other options?

Many thanks in advance for the assistance, MG's has always been my site choice, you guys rock.