I have a 2651 that is failing to boot through. I do not have the true story on whether this was an upgrade gone bad or whatever. Here is the output from the boot:
System Bootstrap, Version 12.2(10r)1, RELEASE SOFTWARE (fc1)
TAC Support: http://www.cisco.com/tac
Copyright (c) 2002 by cisco Systems, Inc.
SPD info: Invalid memory type for Dimm 00000000
I cannot use Ctrl+break; basically the router is stuck at this point (hung, not looping).