Seeing as I've wasted a few hours on this, I thought I'd share the solution.

Friend's PC could not install Java. Verify Java version showed that he had 1.4.2, although I could find no trace of it on the PC. Everytime we tried to download/install Java, we would get this error:

"This installation package could not be opened. Verify that the package exists and that you can access it, or contact the application vendor to verify that this is a valid Windows Installer package.
This installation package could not be opened. Contact the application vendor to verify that this is a valid Windows Installer package."

Tried the alternative downloads (online/offline) and found he had problems downloading (using both IE and Firefox). Tried to download various files from other sites and it was about 50/50 success/failure rate.

When we did eventually get the full offline version, we encountered another error on install:

"Error applying transforms. Verify that the specified transform paths are valid"

Found an article which suggested downloading Microsoft's Windows Installer removal tool to remove Java - this was pointless as Java was not in Add/Remove programs (not even a Java directory).

Solution: I changed the network card!

Good grief!

Hope that helps anyone who ever encounters it.

TEk