Modern file systems like NTFS, HFS+, ext2, ext3, reiser, etc, do not really fragment that much, unless you are dealing with ginorumous hunks of data (like files that range over 50 gigs a piece) so really defragging will not really optimize your HD to begin with because those file systems already optimize them self on the fly.
Not to mention using a active and up to date anti-spyware program to keep the bad programs from installing inside your system,which slows things down even more. Sound kinda redundant to mention it, but when i am helping a friend with their computer, it seems like there is a lack of protection on the computer.
I use Spysweeper 5.5 w/Antivirus. No problems at all.