Are you asking for a computer build that will be able to handle what you need? Based on what I understand of what you have said so far:
Hardware Make : cheapest possible
Processor and RAM : cheapest possible. 1GB memory for windows 7 and 256mb for windows xp
Hard Disk Type : SATA
Disk Size : cheapest possible
Backup Disk Size : what do you need a backup disk for? same size as your main disk.