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It's NOT a good idea to leave your email address in a post anywhere on the web - there are malicious people and malicious programs that are constantly looking for email address to abuse. You should delete it.
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Why do you need to find that file ?
It's NOT running - it can't slow down Windows.
Defrag doesn't find de-fragmented files - it checks to see if files are fragmented and if they are it attempts to make them one continuous file.
Running Defrag on the partition Windows itself was installed on cannot improve the performance of Windows any more than about 10% on modern computers, and that's temporary.
If Windows is running more than 10% slower than it once was, you have problems other than files are fragmented.
$NtUninstallKB16489$\ is probably a normally hidden folder in C:\Windows, as in,
Windows Search doesn't look for system or hidden files by default - you have to select that when you search.
To see the hidden files and folders in Windows in My Computer or Windows Explorer, you have to change default settings in Folder Options.
Control Panel - Classic view - Folder Options - View tab
Hidden Files and Folders -
Click on the circle before Show Hidden files and folder to place a dot in it.
You may also need to click on the box before Hide protected operating system files to remove the check mark
Optionally you can click on the box before Hide extensions for known file types to remove the check mark
Click on OK at the bottom of the Folder Options window
By the way, if you have installed Microsoft's add on Windows Search or Windows Desktop Search, both of those have BUGS that will cause EVERYTHING in Windows XP to run slower on many people's systems.
If Windows Search or Windows Desktop Search is listed in Add or Remove Programs, I recommend that you UN-INSTALL IT !