You can not defrag files that are currently being used/opened, this includes system files. Defragging doesn't always increase the computer speed
To me I find the main cause of a computer "slowing down" and how many programs are running on startup and always running. it is best to first uninstall old programs you don't use anymore and maybe run a disk cleanup.
also running msconfig and going to the startup tab, you can then disable from programs automatically starting up.
the amount of memory can effect the computer proformace too.
hope this helps. i have a few more suggestion but i won't give you a overload of information :-D
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