Not knowing exactly what happened when the "CPU fan fell off" it is hard to diagnose what may have happened within the PC. Added to that it is difficult not knowing what was done and by whom when the "new one" was "sorted". It would also help to know if the SATA drive that can't be found is a second internal drive and if the DVD drive is IDE or SATA.
I am presuming the C: drive is fully functional and you have no problems booting into XP and that you get no messages.
It is possible the connections for the two drives on the motherboard were disturbed/disconnected. I suggest the first thing to do is watch the screen on boot up if the BIOS information (the white script on a black screen before the XP loading bar) is visible and see if the two drives are shown - this would be under a heading such as 'IDE Drives' with a list showing hard drives and CD/DVD drives. If this screen is not visible you will need to go into BIOS (generally by pressing when you first switch on) and see if the drives are recognised there. If they are not present it is possible they are not connected properly to the motherboard or the leads are not correctly installed at the drives. To correct this you will need to gain access to the inside of the PC.
The link shows how to check the connectors are fitted correctly at both ends. You must ensure mains power is disconnected before starting the work and remember that if the PC has been running there will be hot components within it. link text
If you still have problems post back.