You replace the psu(power supply unit) but make sure you get get the correct voltage etc typically an old comp has 250 volts and a newish one p4 athlon has 300 -400 volts
Cost about 10 15 poundsish
best bet is take the old one out by undoing the 4 screws at back that hold it in place itll tell you volts watts etc
get a match for it
and take it somewhere for a replacement or look on the internet
Make sure you have the right connectors tho
A p4 replacement has an extra lead that an athlon doesnt
its a white plug thing
the old type are universal
Hope this helps
Regards mark
Mark Corby
August 2005