The first is the minimalist solution: don't overflow your computer with unnecessary applications, files, documents, media, and anything else. Computers inevitably slow down over time not because of overuse, but because of irresponsible use. The computer in the picture above could probably fulfill all of your basic computing needs as fast as your current netbook. Sure, it probably has megabytes of RAM instead of gigabytes, a processor anywhere from 100-1000x less powerful than yours, etc. But this computer, brand new, may in fact out perform your old laptop or desktop that's been muddled with saved up files over the past years.
If you want to make your computer run just as fast as it was when you first bought it, there's a few simple steps to follow: first, delete all of the garbage. Old useless documents and media, downloaded emails, anything that you don't need. Get rid of it, and delete it permanently from your garbage bin. Then remove all software that you don't use. Don't use iTunes or Adobe Reader? Get rid of it. Whether it's free or paid software, it shouldn't matter, delete whatever you don't use. Paid software can always be reinstalled and redownloaded.
Next, organize everything you have, and get whatever you can off your machine. My laptop, for instance, is a relatively weak 12" ultraportable. The only files I save on it are on my Dropbox, and any programs I use, which is Microsoft Office 2010, FoxIt PDF Reader, and a few others. I've had this laptop for nearly nine months now, and it runs just as fast as the day I first got it. My previous laptop, an IBM/Lenovo T60, was managed identically for two years, and I never had a problem with it in terms of speed. All my files are saved in my Dropbox folder and stored online. This not only keeps me organized, but also ensures all my documents are available on all of my computers.
If you can, I'd suggest completely reformatting your computer with your operating system disc. This will remove all the crapware collected over the years. This isn't always an option, but I recommend reformatting if the source of slowness cannot be determined, or if it's simply faster than manually removing software. The last steps are a bit more complicated. Click the start button and, in Windows 7 or Vista, type "msconfig". In XP, hit "Run", then type "msconfig". A window will pop up. Click on the startup tab and look at the list of applications. All of these are applications that start when your computer is turned on, and these are typically the biggest causes of slowdown. Remove anything you don't need, such as Adobe Acrobat, Google Update, iTunes, etc. Many of these applications you don't actually use normally, but they'll still slow down your computer.
More experienced users can then go to the services tab and remove any services they don't need running. Be warned, first click the "Hide all Microsoft services", then remove anything unnecessary.
Now, just defrag your hard drive, and you should be running good as new!
Oh, and the second solution? Well, on my laptop I run Windows 7 with a 1.3GHz dual core CPU and 3GB of RAM, and I follow the previously mentioned method. On my desktop, I run Windows Vista (gasp!) with a 3GHz quad core CPU, 8GB of RAM and a high-end gaming videocard. It runs smoothly after a year and a half of heavy use, gaming, lots of media storage and sharing, etc. Why? Because the machine is so powerful.
So the other solution is to just have a super powerful machine, just like me!
