I have a strange problem.  Occasionally my system fails to boot to Windows after shutting down unless I unplug the power cord for about 30 sec.  After I push the power on button on the PC I get the initial Compaq screen, but that's as far as it goes.  I do not hear or see any disk activity, and after about 2 min I shut the PC down by again pressing the power button.  I have to unplug the power cord, wait for 30 sec, reconnect the cord and then repower the PC, and then it boots to Windows just fine.  It is a Compaq Presario SR1220NX running Windows XP SP2.  Any thoughts?

Check start\control panel\administrative tools\event viewer.
The various logs should show what errors are causing the boot problem.

   Are you sure that you have to unplug the power cord for 30 seconds?  I do not know what that would accomplish.  Have you tried just waiting 30 seconds and then restarting the computer?
   Sometimes Windows does fail to boot up.  That sounds to me like what is happening to you.  Restarting the computer should solve the problem.

I encountered a similar problem with a Compaq.
You may have " capacitor desease" open your PC and inspect your motherboard with a flashlight and read
http://en.wikipedia.org/wiki/Capacitor_plague
if you do have this problem
I suggess you change your motherboard.