The current average price of replacing a hard drive is around 200 dollars. This can be done through major retailers or a private chain.
It depends on how much information can be stored on the hard drive. The larger the storage capacity of the hard drive, the more $$ it's going to cost you.
No it does not. As long as the motherboard has the proper connectors for the hard drive you are installing, IDE or SATA, you will not have to replace it. If the connectors are different, then return the hard drive and get one with the proper connection type.
operating system
the hard drive should be set to master...
I would suggest installing windows on one hard drive, then installing ubuntu on the same hard drive. Use the other hard drive for the backups (partition it).
Installing the program to your hard drive
yes all hard drives must be reformatted before installing an operating system.
without a hard drive your PC wouldn't work at all. so you must have a hard drive, whether its internal or external...
Defragging the hard drive
yes you do.
You normaly only need a srew driver
configure the bios settings to auto detect the master hard drive
No. Installing Linux is no more destructive than installing Windows.