Are you happy enough to open the case and mess around inside your pc ?
There is a jumper, ( a small piece of plastic with a connector inside it ), that is usually next to the small round battery on the motherboard.
It should be marked CMOS, hopefully.
If you know the make and model of your motherboard you could have a look on the manufacturers site to see exactly where it is as some manufacturers place it away from the battery.
Make sure that the pc is unplugged from the mains socket or that the socket is switched off before touching or moving anything inside the pc.
There are two positions that it can be placed in, it will be in the normal run position now but if you pull it off and connect the central and the other end pins together with the jumper for a few seconds that will clear the memory of the CMOS chip and when you replace the jumper in its original position it will have set the CMOS back to its factory settings.
While you are inside the case you can check that the IDE cables, ( the wide flat grey ribbon ones ), are firmly pushed home into the back of your cd drives and also into the motherboard itself.
Hope this helps