Thanks for your comments folks - I'm glad you found the info of interest.
I should mention that I experienced an initial problem getting the software to run. I got a message telling me that the Java Virtual Machine failed to launch. This is a known problem and there is an easy workaround. This page covers all know issues:
Zerene Stacker Known Issues
Geoff - Your problem sounds like it
could be memory related. If you're running with 1Gb or less of RAM you
may have to reduce your image size (not file size) before stacking. Cropping is best of course but if you experience problems and you can't crop it may also be worth trying to resample down to a smaller image format (but keep it as large as you can of course).
Bruce
Bruce