On most people's computers, several small programs will start every time the computer starts, and keep running as long as the PC is on. Press Control-Alt-Delete once and you will see a list of the programs running on your computer. Everything except Explorer and Systray is unnecessary. If you aren't using any programs, but there are programs listed besides Explorer and Systray, then your computer has some programs that start every time it starts. They will make your computer start up slower, and can cause errors while you are using other programs. These little programs can come from installing software (like RealPlayer) or they can be installed by your computer manufacturer. Wherever they come from, they probably aren't doing you any good and are slowing down your computer. You can close them in the Ctrl-Alt-Delete menu, but they will come back when you restart your computer. This page will tell you how to keep them from starting. If you follow these steps, you will still be able to run the programs if you need them, but they will not start on their own. If you want to have an antivirus program running in the background, you should check to make sure you aren't disabling it (If you do disable it, you will still be able to scan for viruses, the program just won't be doing it constantly).
The first thing you want to check is the Startup menu. Click the Start button, go to Programs, then Startup. Anything that's in there can be deleted. The files in the Startup menu are just shortcuts, so deleting them will not delete any programs, it will just keep the programs from starting every time the computer starts. To delete the shortcuts, right-click the Start button and click Explore. A Windows Explorer window will come up. Go into Programs (in the right pane of the window), and then Startup. Delete everything in the Startup folder, unless you know that you want a certain program to start when your PC starts. There may be shortcuts associated with a useful program, like Office. You can still delete them and the program will work fine. If you don't know what a file is, delete it. You can't hurt your computer by clearing out the Startup folder. If you deleted some files, restart your computer. Press Ctrl-Alt-Delete and check for unnecessary programs. If you only see Explorer and Systray, you're done.
If you cleared out the start menu and you still have unwanted startup programs, you will have to edit the Registry. It's not as hard as it sounds, really, as long as you don't delete the wrong things. Go to Start/Run, and type in regedit. You will get a window that looks sort of like Windows Explorer. In the left pane, there will be about eight folders, with names like HKEY_CURRENT_USER. Double-click HKEY_CURRENT_USER. More folders will appear under it. Open the one that says Software, under that open Microsoft, then Windows, then CurrentVersion,then Run. In the right pane, you should see a list of "keys." Each key in the Run folder is like a shortcut in the Startup folder. To the right of the key's name, you will see a path, like c:\program files\useless\useless.exe. Delete all the keys, but if you have any programs that you want to start when your computer starts, look at the paths and make sure you aren't disabling them. If the path goes to a program you need, like Office, you can still delete the key if you don't want the program to start when your PC starts. After you do that with HKEY_CURRENT_USER, do it with HKEY_LOCAL_MACHINE. In HKEY_LOCAL_MACHINE, besides the CurrentVersion\Run folder, there will be shortcuts to startup programs under RunOnce, RunServices, and other things that start with Run