Some computers just don't have enough RAM.
The programs you use every day run off your RAM - which stands for "random access memory," and is sometimes cited as DRAM, with the "d" for "dynamic."
It's common that, after a year or two, bigger programs and the tendency we all have to run a gazillion programs at once catch up with us.
For some people, adding a bit more RAM is a cheap and easy fix for the slowdowns. Unfortunately, figuring out what kind of RAM your machine uses can be a headache.
Save yourself hours of research with the Crucial Memory Adviser. It tells you how much RAM your computer has and how much it can use. It will even tell you what types will work best in your machine.
Input your computer's make and model and the site tells you everything you need. If you're not sure about model numbers, there's a version you can download that will figure it out for you.
To download, click the System Scanner button on the front page. There is a video you can watch about downloading and using the System Scanner tool.
The Adviser recommends specific brands of RAM, too. You can buy them right from the site, but it may be cheaper to search around once you know what you're looking for.
As far as installing the RAM, do-it-yourselfers can add their own; others will need to take it into the shop.