How?
1. The quick fix that only lasts until you restart your computer is to press Ctrl-Alt-Del and to close any programs not currently used. The few programs you *want* to keep running are explorer and systray. These two are required. Depending upon your computer, most of the other applications may be unnecessary. Right down the names of the programs that are running when you press Ctrl-Alt-Del.
2. Many of these programs will show up as icons in the bottom right hand corner of your computer screen, by the clock. You can usually click, double click, or right click to access options on those programs. Often times you will find a disable option or something like that (exit usually just exits until your computer is restarted). Usually the programs that have these icons aren't very useful and can be removed to free up memory and speed up your computer.
3. Now you can go to your Startup folder on your Start Menu by right clicking on the Start button and going to Explore. Double click on Programs, then double click on Startup. Look for any programs that are not necessary, click on them, and press the delete key (you may want to instead drag and drop them to your desktop in case it later turns out that you need them (then you can just drag them back).
4. That only gets rid of part of them. The next process is a little more complicated. To be safe, you may want to Backup your Registry. Click Start, Run, and typeregedit in the box. Double click on the folder called HKEY_LOCAL_MACHINE, then do the same for Software, then Microsoft, then Windows, then Current Version, then Run. These are the programs that run at startup. Before you delete any of them, be sure that you don't need them (if you backed up your registry, that shouldn't be a problem - you do need to have SystemTray though). To delete an unwanted program, just click on the Name, and press delete. You may also want to check in these folders below the Run entry:
RunOnce
RunOnceEx
RunServices
RunServicesOnce
5. Those steps should take care of most of the background processes, effectively speeding up your computer. Tools like the Microsoft Office shortcut bar can be major memory hogs, so I would also advise not using them. Good luck!