I am not a fan of Windows, and everyone knows it. I have to run Windows (XP, if you care) for some time, and so my laptop has Windows on it. One problem with Windows is that it becomes very slow after running for a few days. Do you know why? Because applications that we keep running eat up all the memory and Windows doesn't have enough memory to keep running.
When you see that the apps are getting slow, open Windows Task Manager (by right clicking on task bar and choosing Task Manager from the menu) and see how much memory your computer is using. In this particular screenshot, about 1.42GB memory is used by Windows and all currently running applications.
If it's using more memory than what your computer actually has, your computer will be slow and you should free up some memory. Freeing memory is easy: just restart the apps that are consuming a lot of memory. If you are like me, most of your memory would be taken up by browsers.
Go to the Processes tab of Task Manager and sort the list on descending order of memory usage. You will see something like this:
You can see from the image that Chrome is taking up a lot of memory. It's natural because I never close my browsers. You will see several entries for Chrome in the task manager because Chrome starts one process for each tab, extension, plugin, etc. Close the unnecessary tabs and restart Chrome. (Your Chrome restores all previously open tabs, right?) Restart any other app that has been running for a long time and consuming a lot of memory. This should make your computer considerably faster.
Maybe you don't have to do so much
Sometimes, one Chrome tab could be using a huge amount of memory and you could probably close only that tab and keep going. Chrome has its own task manager. Chrome's task manager, when sorted based on memory consumption, shows which tabs take up lots of memory. In my computer right now, Gmail alone is consuming 82,500KB of memory (which is a lot). If I don't need Gmail to be open now, I can close it and I will get this 80-odd MB's of memory back.
Of course, this technique may not work for you if your usage patterns are different from mine. By doing this once in a few days, I have been able to run my Windows machine for more than 17 days now, which is impressive.
 It might sound ridiculous, but your computer can really use more memory than you actually have.