AgentZero said:
"Windoze XP Home Edition for under $100 as long as you keep your receipts for mother board, hard drive, RAM and a CPU."
For reference, you only need that information to get a free (plus S.H.) upgrade to Vista from Microsoft, as part of their "don't hate us cause we released a month after Christmas" campaign. You don't need to actually buy such hardware; you can enter whatever serial numbers you like (even 0), apparently, and still get the free upgrade.