Could you post some system specs please? Mainly the motherboard, cpu and ram. It's always helpful to include your specs when having a hardware issue. You can get this information using a utility such as cpu-Z :http://www.cpuid.com/cpuz.php
I'm thinking at this point a BIOS flash may be in order. It's best to do it using a floppy or a bootable CD-R. If you know your motherboard manufacturer and model you can take a look at some of the BIOS updates and the problems they address.
Try these steps for a good CMOS clear:
*Power down and unplug power to tower
*Push the power button on your tower to eliminate residual power
*Move jumper to clear position and leave it there overnight
It's somewhat of a long shot but believe it or not sometimes it actually does work.
The jumper on the motherboard probably didn't come off on it's own. The jumpers are typically snug on the pins. So it's possible it was never there in the first place.
Flashing with a bootable CD-R is possible but you might want to *try* and eliminate all other possibilities before taking that step.
I also want to verify that this is actually an AMD Athlon XP 3000+ and not an AMD64 3000+.