You can explicitly tell Eclipse where to find it. Open eclipse.ini and add the following lines to the top of the file:
-vm
/absolute/path/to/jre6/bin
Update: I just nailed down the root cause on my own Windows machine. The GlassFish installer complained with exactly the same error message and after digging in GlassFish forums, the cause was clear: a corrupt JRE install on a Windows machine. My JRE came along with the JDK and the Java 6 JDK installer didn’t install the JRE properly somehow. A DLL file was missing in JDK’s JRE installation. After I reinstalled the standalone JRE from http://java.com, overwriting the old one, the GlassFish installer continued and also Eclipse was able to start flawlessly without those two lines in eclipse.ini.