Many programs attempt to launch themselves when your computer starts. Anti-virus programs, instant messengers, video drivers, mouse drivers - the list goes on. Individually, these programs are usually very easy on system resources, but when put together, they can sap a great deal of your PC's performance. By cleaning them out, you can free up RAM for use in other tasks, and also reduce the burden on your CPU.
To clean out unwanted applications, first take a tally of what unneeded programs are currently running on your computer. Anything that does not provide a vital service can be cleaned out. For example, it is probably a good idea to keep your anti-virus and firewall programs running at all times, but having instant messengers running when you don't plan on using them will only sap your system resources.
Once you have your list made, open up MsConfig. You can do this by navigating to the Run application on your Windows menu, typing in msconfig.exe, and hitting the Run button. If you have a Vista machine, you can simply type msconfig into your search bar in the Windows menu. Once you have MsConfig up, choose "Selective Startup" in the general tab. Then navigate to the startup tab. Here you'll find a list of programs that start when your computer boots. Uncheck anything that you have determined you don't need. Then click Okay to exit MsConfig and save your changes. You can choose to reboot immediately, but you don't have to.