Starting with GNUCash 2.04.09 through the present release (2.04.11) GNUCash does not run if Lilypond is installed.  With Lilypond GNUCash will not start.  It fails without creating and trace information.  Windows (my system) reports a conflict or a problem with libguile.  My research on the Web indicates that this problem has occurred before and seems to be related to an upgrade in the guile system.  It is my understanding that one program (especially those as conceptually unrelated as Lilypond and GNUCash) should not affect the operation of other programs on a system.  In the worst case even if Lilypond is the cause of the problem, this problem still exposes a weakness in GNUCash.
Lilypond works
GNUCash will not start
GIMP works
If I uninstall Lilypond, then GNUCash works
details:
GNUCash 2.04.11
Lilypond 2.14.02 or 2.15.41 (yes I realize 2.15 is unstable)
Windows 7
Windows problem signature:
Problem signature:
  Problem Event Name:	APPCRASH
  Application Name:	gnucash.exe
  Application Version:	0.0.0.0
  Application Timestamp:	50030488
  Fault Module Name:	libguile-17.dll
  Fault Module Version:	0.0.0.0
  Fault Module Timestamp:	4ff506b4
  Exception Code:	c0000005
  Exception Offset:	0002cc2f
  OS Version:	6.1.7601.2.1.0.768.3
  Locale ID:	1033
  Additional Information 1:	0a9e
  Additional Information 2:	0a9e372d3b4ad19135b953a78882e789
  Additional Information 3:	0a9e
  Additional Information 4:	0a9e372d3b4ad19135b953a78882e789