I tried running Minecraft Forge (in eclipse) again, on my old Windows XP computer this time, but with exactly the same result.
Environment:
Operating System: Windows XP 32bit
Java: jdk-7u71-windows-i586
Eclipse: Version 3.4.2
ForgeGradle: forge-1.8-11.14.0.1268-1.8-src
Out of curiosity, what are the exact versions of the java runtime environments Minecraft Forge (in eclipse) is known to start up correctly.