Computer stops when rebooting, emitting a repeating slow beep over and over.  Pushing reset button allows it to reboot most of the time...

This was a tricky one for me.  I build my own computers and Lord knows how many things can go wrong.  Well, this problem (of my own making) was so dumb I'm almost embarrassed to admit it! 

Last month, I installed a new larger memory stick.  After a few weeks, the computer started locking up right after shutting down, beeping a repeating long tone.  If I pressed the reset button, it would boot fine. (NOTE:  Some motherboards may require unplugging the power to get the same effect, since their reset buttons don't completely shut off the computer and clear the memory.)

Motherboards require that if you only fill one memory slot, the stick of memory must be installed in SLOT 1.  The slots on the motherboard have some numbering configuration indicating the 1st, 2nd, 3rd, etc. slots. 

MY BAD?  I installed the single stick of memory in SLOT 2 instead of SLOT 1.

Frankly, I have no idea why the computer ever worked with the memory in the wrong slot... it shouldn't have.  Also, it took a while for the problem to appear.  Equally mysterious.

By moving the memory into the correct slot, the computer now starts fine every time.  Go figure...