New computers often come with a whole bunch of extra crap installed. It can be hard to figure out what programs you should keep and what you should delete. And I imagine that a lot of computer owners are not likely to delete anything, thinking that the software must be useful since it came with the computer.
In fact, the crap that the retailers and software companies install on your computer can affect its performance — slow it down and drag out your startup. And even if your computer is on the wrong side of new, it can probably do with some decrapification, especially if you notice that it is taking longer to start up or do trivial tasks.
The PC Decrapifier will take the crap off new Windows XP and Vista computers, and might even help you remove some things from your old computer. (The author says it is unlikely to be very useful on older computers, but I was able to get it to remove a couple of things from my startup.)
For older computers, have a look at this article from PC World (where I originally found out about PC Decrapifier): 20 Tools to Get the Junk Off Your PC. Be sure to check out the user reviews of the software that they recommend before you download anything.