@Tatjahna,
Try a tool called RegCleaner, download it
here.
It will work under Win9x, WinME, WinNT, Win2k and WinXP.
It will run a scan of the register and show, among other things, register keys linked to non existing programs, files, etc. It's a free prog and non invasive, it has proven itself reasonably useful to me in the past.
If, it works out for you, it will save you a lot of time versus a manual search.
Make sure to run the register scan on MANUAL and NOT auto mode, also make sure and have it make backups of any reg keys you remove, so you can restore the keys if necessary, that is the default, if I recall correctly, but just double check the settings before running it.
Good luck.