When you make any changes to the Registry, you should first make a back up copy.
Your utility, Regseeker 1.52, has the ability to backup the registry and backup changes that it makes in case you need to reverse your action. Use it.
In addition, it would be a good idea to make a separate, complete backup of the Registry before taking any automated action with Regseeker 1.52.
Open the Registry Editor (go to Start/Run and type regedit and press Enter). Pull down the Registry menu and choose ExportRegistry File…. Navigate to where you'd like to store the backup file, type in a name for the file, (for instance, today's date: RegBackup112606.reg) select All under Export Range, and click on Save.
To restore the entire registry from this saved file (if you need to do so in the future), simply double-click it in Windows Explorer. Note that the REG file will be quite large, possibly a couple of megabytes. Be patient while Regedit saves the file, and if you restore, give it a few minutes to complete that job as well.
As to your question about "unused file extensions"……. I have not used Regseeker 1.53, but most automated tools work in a way to remove unused files with those file extensions, and not the machines ability to recognize the file extension itself. Additionally, some applications, like windows explorer or mediaplayer "watch" their own file extensions and have the ability to recreate them as needed. But some third-party applications may not have that ability. Therefore, if you remove the file extension itself, the specific application may be rendered useless.
In general, the "unused file extensions" are not taking up much space, nor time of Registry read, and can be left in place.
You are apparently working with a tool that you are not familiar with. And you propose to use it on a portions of your machine that if "broken" will render your machine inoperable without reinstallation of the Operating System. You could lose alot of data and time of reinstalling system, drivers, and applications.
I've seen HooverDesk RegSeeker recommended by many well-known helpers in various forums, but remember that those folks already know what they are doing.
It is not unheard of to find a few hundred unused/invalid/uninstalled/missing/obselete entries in a registry that has been running for a while, especially if the owner/user has installed and uninstalled a variety of applications over time.
As with most "repair" work, the old adage applies: "If it ain't broke, don't be fixin' on it."
But you'll still probably decide to move forward and try this Registry Cleaner out.
And many well respected resources do recommend Registry Cleaning for a variety of reasons.
Bottom-line, is that it is your machine. Pursuing this course of action is likely to help you learn more about your machine. That's a good thing.
Learn as much as you can about Registry before taking action.
At the very least, read and understand this good but rather dated article:
http://www.microsoft.com/windowsxp/using/s…_03march17.mspx
Best Regards