Thanks okee,
Your advice did not work directly, but it lead me to the solution.
I uninstalled, deleted everything, rebooted, reinstalled, rebooted, and still no compiling.
Then I took it a step further, and did everything above, plus searched the registry for all references to GLBasic, and deleted them.
I then reinstalled, and it works as expected, although I had to re-enter my license key of course.
@Gernot
I don't think I need more help, it works on my machine now.
Do you know why the registry may have prevented the new version from working on my machine though?
Is this perhaps expected in XP, or was this unusual, and worth investigating what went wrong?
@MrTAToad
Your post had me worried, I thought perhaps that would be an incompatibility for me as well

I am very glad that wasn't the case, I can't afford a hardware or OS upgrade right now.
Thanks,
Dave