Many programs that people install, especially multimedia and communications programs, set up at least a portion of the program to "run at startup", which simply means that when your computer starts, the program starts too. They do this for a number of reasons:
Some of these are very helpful, but many of them are just consuming resources on your computer, and not providing any real benefit. Most of the time, when someone tells me their computer seems slower than it should be, I can find a half dozen programs that are running in the background which aren't really being used.
An easy way to identify some of these programs is to look in the area known as the "System Tray". This is the area down by your clock, that usually has a handful (or in some cases, dozens) of icons in it. These icons can include your Volume Control, Network Status icons, Antivirus programs, Instant Messaging, Media Players, etc. Usually, you can point your mouse at one of these icons, and a "tooltip" will pop up to tell you what it is. Often, if you right-click the icon, you will get a menu of various things that program can do.
This menu will likely have an "exit" or "close" option, but that is only a temporary help, because if the program is set to run when your computer starts, it will just start back up the next time your computer is restarted.
A second place to look is the Windows Task Manager. You can get to this using the infamous CTRL + ALT + DELETE key sequence (press all three together). The Task Manager lists all the processes that are running on your computer. You should not be alarmed by the number of processes running - in some versions of Windows it is perfectly normal to have 15 or more tasks running at any time. You should look through the list, though, and find any programs or processes you feel should NOT be running. The processes do not always have descriptive names, but searching the internet for the name of a process should give you some information about what it's for and how useful it is.
The Task Manager gives you the ability to kill a specific process. Use this power with great caution. Killing a process can cause unexpected results, especially if you are not sure what that process is actually doing. And once again, killing a process is only a temporary help if the process is configured to start up again when your computer restarts.
So, how do you prevent them from starting up in the first place? There's not a catch-all solution, because each program is different, and there are multiple ways for a program to start itself up. There are a few common things to try, though.
Note: You should not disable programs that offer protection, such as antivirus programs. These are in place for your own good, and the benefit they offer outweighs any impact on performance.
If you search through the menus for a program (these are usually File, Edit, View, etc. along the top of the window), you can usually something for "Options" or "Preferences". This is where you configure the program to your liking, and change whatever options it has available. Look there first to see if there is a checkbox to "Start when Windows starts" or "Load Automatically". Turn that option off, and restart your computer to see if the program starts itself up.
In the Start menu, under Programs, there is a folder called "Startup". Any program with a shortcut in this folder will be started automatically when your computer "starts up". You can simply remove shortcuts from this folder to prevent the program from starting every time Windows starts. Removing this shortcut does not uninstall the program, so it is still available if you need it - you are just preventing it from starting automatically.
If a program has no options to prevent it from running at startup, it is not very well written. You must decide how valuable this program is to you, and if it's use is worth the impact it may have on the rest of your system. If not, you may want to remove it completely.
See Also: Removing Unnecessary Programs