I tried setting the permissions on the registry key, but it wouldn't let me change them.
I also keep a system image on a spare hard drive (all my data is kept on a completely separate drive) so I tried restoring from that backup, but when I tried to update Windows Live Mail, I got the same error messages again, lol.
In the end, I formatted the partition and did a reinstall. I keep up to date installers for all my apps on a different partition, so it wasn't a major hassle to reinstall Windows then reinstall all my apps. All this hassle just to update one program, lol. Oh well, it's all sorted now, so thanks for your help.