[news.eclipse.newcomer] Bad version number in .class file

[rodrigobraz@xxxxxxxxx (Rodrigo de Salvo Braz)]

Hello,

I had a project using SWT and JFace (stand alone) in my PC. I have now 
moved to a Mac and moved my workspace to the new computer and a new 
Eclipse installation. When I opened Eclipse, some jar files in the Build 
Path had to be updated to whatever jar files my new Eclipse installation 
had (the version is part of the jar file name -- that was the one thing 
that changed). I did that and the project compiles. However, when I run 
it, I get:

Exception in thread "main" java.lang.UnsupportedClassVersionError: Bad 
version number in .class file
	at java.lang.ClassLoader.defineClass1(Native Method)
	at java.lang.ClassLoader.defineClass(ClassLoader.java:675)
	at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
	at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
	at java.net.URLClassLoader.access$100(URLClassLoader.java:56)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:316)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:280)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
	at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:374)

When placing a breakpoint at the exception, I see that, at the
Launcher$AppClassLoader(ClassLoader).defineClass(String, byte[], int, int, 
ProtectionDomain) line: 675	
level,
arg0 is "jface.test.JFaceTest" (id=26)	

My new Eclipse installation version is
Eclipse SDK
Version: 3.4.2
Build id: M20090211-1700

I don't know precisely what version the PC installation was, but it was 
recent too, 3.4.2 or 3.4.1 at the earliest.

Can any one give me a hint on this, please?

Thanks,

Rodrigo