windows - Positive effects of a OS re-install

I have a slow computer, and I can uninstall unused applications, empty the cookies, scrub the registry, do a disk cleanup, and do a defrag, but it is still just kind of slow.
If I do a Windows re-install, same OS, and reinstall all of the exact same programs, the computer is pretty fast again.

What exactly is the difference between the fresh and old OS installs?

Answer

Serverfault question ref: System degredation - does Windows slow down over time?

Mark Russinovich's take on The Case of the Slooooow System
describes how to analyze a slow system.

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For personal experience, before a reinstall you could do these things...

Backup all data files (basically leave just the bare windows installation intact)

1. While I discuss the Windows boot partition; you could clean up the other drives similarly
   
   
   
   • Uninstall all unnecessary applications
   
   
   • Cleanup application caches
   
   
   • this would be browser caches and such
   
   
   • Use autoruns to check what starts with your system
   
   
   • take a call on what is required, stop or uninstall as you feel fit
   
   
   • Check if you can cleanup the Windows installation a bit more
   
   
   • Use RevoUninstaller to cleanup remaining bits
   
   
   • Use CCleaner to clean registry
   
   
   • Use JkDefrag and move all files to one end of the boot partition (option '-a 6')
   
   
   • Disable page file, reboot, re-enable page file to the maximum suggested size
   
   
   • use the custom setting to freeze the page file directly to that size
   
   
   • You could run JkDefrag once again with defaults ('-a 3'),
     if you feel like it after you restore your backed-up data files
   
   

Restore backed up data files.
Check the feel of the system for a couple of days.
You can always go ahead with the re-install/recovery if required after that.
Your data backup would already be in place.

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I have used the PageDefrag and Contig tools referred by Molly in the other answer.
These are great tools. But, I have a few opinions,

1. It is not always useful to run Contig on multiple files of a partition
   
   
   
   • I have found JkDefrag options doing a better job (nothing against Contig here)
   
   
   • There are times when removing a Page file and creating a fixed size one in a defraged partition works better than PageDefrag (which cannot do much if your partition is nearly full to start with; again, not a problem of PageDefrag)
   
   
   • In the interests of keeping more free-space on the boot partition,
     you might consider moving the pagefile.sys file to another drive (not partition),
     It is suggested by some.