Windows XP is creating a backup of every patch that you install for the operating system. The files are placed in the Windows directory and start with $NtUninstall. Microsoft is creating those backups so that the user can easily uninstall the patch if problems should arise.
Those backups do not get uninstalled after a set period of time though and can quickly accumulate to a size of 200+ Megabytes. The files can be simply deleted in the Windows folder. Users who prefer to use a software can use a tool called Remove Hotfix Backup Files which removes the files automatically.
It should be save to delete those backup directories if you have worked with Windows for several days after installing the latest patch from Microsoft without encountering a problem. Only patches that affect Windows XP will be removed by the software, patches installed for other software like Windows Media Player or Internet Explorer will remain.
Users who want to be on the safe side make a backup of the folders by either burning them to a CD or DVD or moving them to another folder or drive. If something goes wrong the folders can simply be moved back to the Windows directory.
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My personal favorite tool to accomplish this task is Ccleaner. It cleans these files, and many, many more. Check it out: http://www.ccleaner.com
I’ll second CCleaner.