Well, if you're planning on installing a new HDD, they you do realize you'll need a full version or at least another OEM version of windows.
Install new HDD (Make sure jumper in on master)
Old HDD (Change from master to Slave)
Connect both (VIA) IDE - On proper Master/slave
Plug in a molex.
Boot to new HDD with OS.
If all has been done correctly, you should be able to access the old HDD. Transfer all data that is needed.
Format old drive if you want.
Then buy yourself a External IDE HDD enclosure. These normally will connect to your PC (VIA) USB. Then you'll have additional Storage.
Or you could just buy a new HDD and an Enclosure. Save you some trouble.