First of all a hard disk dating back from 2002 would be PATA (IDE) and as such would be a performance restriction in a new pc when compared to the new generation high speed (SATA) drive with a decent amount of cache.

The only reason to build your own PC is if you want something specific, I generally do this for either high end pc's where performance is the consideration rather than costs or for a SMP (Symmetric Multiprocessing)
Otherwise you would be better buying and saving the money