I'm a bit more extreme, I tend to reformat my HD and reinstall everything I want. I do this at anything between 6 and 12months. Yes it is extreme, but it gets rid of a lot of guff that gets put in strange places on your system.
Often this is prompted by a crash or something, but I like to have a good clear out.
In order to make this a little less tiresome, I have a partition purely for programs and OS, all my data is in other partitions and I don't reformat that. I do do a defrag tho, and this is usually at the same time once I have the OS up and running again.
This approach makes it more important to backup everything you download regularly. I keep all security updates etc on CD for future use.