You know how your computer slows down over time...it was fast when you got it and brought it home but slows down slowly until you're wondering what happened? Well, there's more than one reason for that. One reason is that software continually gets bigger, so new versions of software will run slower on the same computer. The other reason is that your computer collects junk, your disk gets fragmented, stuff collects that you're not aware of simply because you download and install software and then remove it, and it leaves behind piles of files in various places that just collect.
One of the best ways to clean your registry or improve your system's performance is to re-install your computer. This is especially true if you've had a serious virus, but in any case, after a few years of collecting stuff, any computer will benefit from a full re-install. It will go back to being the speed that you remember when it was new, although as you re-install the software that you need it will slow down some just from that. But it will be lots faster than before you re-installed.
There are many incremental things that you can do, like defragmenting your disk, making sure that you only start up the programs that you need (msconfig), and just being careful with what you install and remove, maxing out the physical memory in your system, and so on, but a full re-install really has the most beneficial effects.