borthwick,
Thanks for your feedback.
If it is some application installed through some professionally made installation inf or else, you have of course uninstall it accordingly. (Think for instance excel or netscape. You get repercussion all over if you just delete some entries in the registry by brut force.)
If it is just some executable "accidentally" registered when you associate some data file through open-with then "others" (through browsing), then it might be possible but not without care.
[1] Delete the corresponding key in hklm\software\... as I mentioned and
[2] De-associate the data file from referencing it. Safer to do it with File and Folder Option, meaning, all actions involving the application be removed (not only default actions).
[3] Scan through the registry for any reference to it. Note them down, and assessing the risk of deleting those references. (Be very careful though.)
Maybe some utility outthere can do the job safely. But, I am not using many of those. Somebody here might give you good advice.
In any case, reveal more info so that other here can help. I am on-the-way to a day-off. Above are just a quick thoughts.
regards - tsuji