Oracle Database Express Edition (XE)

I have been working for some time with Oracle Express,, installed on Windows Vista (yes, I know. You have to go with what you have available). Unfortunately it got into an odd state (tablespaces I couldn't remove) so I decided to uninstall and reinstall. I used the uninstaller, but it left a lot of files and other things on the system. I deleted these by hand, but now can't reinstall; the installer tells me I already have an instance on the system, and it can't proceed. I have:
- cleaned all Oracle-related entries out of the registry, with the exception of items that appear to belong to Microsoft (start with microsoft or x86-microsoft)
- removed all files that seem remotely oracle-related
- cleaned all entries out of the start menu
- cleaned all entries out of the environment variables
Has anyone come across this problem? Is there a solution? Or am I going to have to blow away the contents of my hard drive and start again. (In which case I would perhaps go to Windows 7 and run XE on a virtual machine, as recommended elsewhere.)

Thanks for any help!