For the last few months, I’ve been using [IObit SmartDefrag][sd]. It
is a freeware defrag program that keeps your drive running
fast. Basically, you install it and it runs in the background, like a
librarian silently alphabetising your book collection. Your machine
can find and load files faster. Anyway, it feels like my machine has
stayed fast, where I would have expected it to slow down over
time. Anyway, it’s freeware, it seems to be reliable, and it’s kept my
machine fast. Can’t go wrong.

[sd]: http://www.iobit.com/iobitsmartdefrag.html