When It's NOT Necessary to Upgrade Your Computer
Many computer dealers and electronics stores that sell upgrades don't necessarily wanting consumers to know this, but a simple, clean wipe of the computer's operating system will generally restore a PC back to the initial speed it had when the customer first bought it.
You see, as time goes on, a computer becomes like a cluttered desk. You add papers, and move things around, and stack junk on top of junk, and eventually it becomes almost impossible to find anything on your desktop. Your computer works the same way. Every now and then (maybe once a year or so), it's a good idea to just WIPE the desk clean and start from scratch.
Most computers sold in the last several years even come with a "System Restore Disc" that you can insert into the CD/DVD drive and with just a couple of clicks, restore the computer to it's factory settings. This will wipe and reinstall Windows, removing any junk (applications, viruses, spyware, etc.) and most likely will restore your computer to the original speed it had when you first bought it.
Of course, people should back up any important data to CD/DVD before wiping their hard drives, but this is an economical way to get a couple more years out of your computer. In fact, for the vast majority of people who are just browsing the Web and maybe working with Word and Excel, a 5 to 10-year old computer should be just fine for most purposes. (I am currently writing this on a 10-year old Pentium 2 PC running Windows 2000, and it's just fine.)
Now, the guys who sell hardware upgrades don't often want to recommend this to their customers, but I've been recommending it to my clients since the mid-1990s, and it works great. Obviously, if you purchase a new piece of software that has very high hardware requirements (like a new game) or you're physically running out of hard drive storage space, then a hardware upgrade might be in order. However, just to restore lost speed with the same old applications, a clean wipe is the best solution.