if you put the 300gigger as the master you will have to format it. Then install windows from the begining. You just can't take the c: drive which is now the master and has all your programs on it and just make it a slave and exspect xp to work. Why not just make the 300gigger the master on the second IDE channel? Also you have to make sure you computer's bios will support a drive that big.
My advice is to take it to a shop and let them do the work.