For various reasons, a PC I use has Java 1.4 installed and the latest Eclipse Europa requires 1.5. I am unable to update the classpath, environment etc. so although 1.5 was installed, Eclipse would not find it.
The solution was to use the command line option:
C:\eclipse\eclipse.exe -vm "C:\Program Files\Java\jdk1.5.0_15\jre\bin\java.exe"
This forces Eclipse to load with the 1.5 VM.
I placed the command above in a shortcut so it's easily accessible from the desktop.