Originally Posted by Freebase Dali

Defragging helps, but there's other stuff, like disabling background processes you don't really need/use (thereby freeing up more RAM for stuff you actually do use) and cleaning up your registry... also doing general tidying like deleting browser caches and deleting SP uninstallers. A good program to do the general tidying stuff is Ccleaner, which is free. As far as disabling background processes, there are guides on the web which tell you what each running process IS and what it DOES and whether it's important or not so you can safely make a decision to disable it or not. For instance, if you never print from your computer, you could turn off the printing and spooling service. Never do any file sharing within your home? Turn off file sharing. Don't plan on using Remote Desktop? Turn it off. Etc..
The idea is that if you can disable enough (running by default) background processes, you'll go a long way in freeing up space in your RAM for applications you actually use, like Explorer, your browser, your media player, etc.

Of course, you could always buy more RAM... but for the sake of ways to improve things for free, I thought I'd mention it.