Reinstallation is the only approach. Try reinstalling without removing any of the installation, this should run through the installation process but transfer a minimal amount of data and end by correcting the environment settings for the products. IF that fails, remove teh product installation through Add / Remove Programs, delete the product installation directory, reboot and reinstall from the original source media, apply any updates.
In reply to oldmouldy:
Many thanks for all your help, it is working again.