a) new PC usually has OS already loaded. You can choose which OS to buy but will need to pay somewhere for a new OS.
b) I recommend - put the old HD in an external case matching EIDE or SATA whichever the old HD is.
This way will avoid warranty problems opening a new PC and always be handy for backup as well as transferring your old data to the new machine.
c) - onwards - Be prepared to have to install all major apps fron their original CDs as well as other programs which you have downloaded from the past.
New PC? It's a huge wide market and warrants a separate thread, or at least a good read of many posts on the PCA forum