[I know this is reply to old message]
For anyone looking for a great windows defrag program that costs nothing, I suggest this one:
Defraggler found direct from developer here, and has been around a while: Defraggler - File and Disk Defragmentation - Free Download
Why you use it, takes your files which Windows (including any/all programs you run) creates and writes tons of them all the time without you even realizing it, the defrag programs job is to put these programs on your hard disks together in orderly (contiguous) fashion, so that your HD mechanisms don't have to jump around all over the place to find pieces of various files, making it much more efficient.
Users who don't defrag but use most of their hard drive space for whatever, say MP3's or games, Windows without help will use the space available, and due to deleting, writing, rewriting constantly, the actual available spaces changes and the performance of the PC will SUFFER until defragmentation is done.
Another example, surfing the web, you aren't really surfing, you are downloading content (files) and the browser is interpreting them in a user friendly manner, browsers do not have unlimited space for this and depending on how the browser cache is set up, files are getting destroyed and new ones created at a very fast pace over time and where they wind up, is scattered inefficiently. This also includes cookies!
Defrag once a week is what I recommend to a typical user, when you won't be at the PC for a long stretch is ideal, but I use defraggler in the background on partitions I am not using at the time, also.
Defragment tools have come a long long way and are much faster, thank goodness, since hard drives/data has bloated up as well.
If you rarely use your PC then, you rarely need to defrag.