This does depend on the nature of the original setup and the discs that came with the machine.
If your are fortunate enough to have a full XP disc
[I assume the original system is XP]
Then try this- indeed try it any way with ever disc you have been supplied with.
Start the machine and when the P.O.S.T comes up press the key it says to access the BIOS.
It is usually delete.
Using the up-down keys select Advanced Bios Settings
Using the up/Down Keys select boot with.
Using the page up/down keys until boot from CD
Press F10 'Boot with changes
Select Y and enter.
Put the disk in and see what happens
various messages will come up depending on the state the disc HD is found to be in
Report back here with that info and we go from there.