Over time, every computer gets more and more junk, er, software, added to its autostart locations. What starts out as a happy user experience, in which a PC owner is pleased with the performance of their new system, can turn sour after few months or years. By then, they are complaining about its performance -- especially Microsoft Windows users. Most blame Microsoft.
In 99 percent of the cases, it has nothing to do with anything Microsoft has added and everything to do with what the user has intentionally installed. (I am a full-time Microsoft employee.) Just about every software vendor out there contributes to the problem. In their zeal to position their product's usefulness and performance, they frequently add one or more of their programs to users' autostart list.
Want to see what your PC automatically runs when it starts up? Check out Microsoft's free utility, Autoruns. If you're unfamiliar with the inner workings of Windows PCs or who haven't run Autoruns before, you'll be amazed. The average computer runs well over 100 autostarting programs, and perhaps 20 to 50 percent of them aren't needed when your PC boots up.
Notorious performance killers: Unneeded programs
I recommend all users evaluate which programs autostart on their PC and at least remove the ones that aren't used anymore. Start with Add/Remove Programs and uninstall any unneeded program. Then run any of the great utilities showing you everything that automatically fires up when the computer starts or the user logs in. Along with Autoruns, I've happily used Silent Runners or Trend Micro's HijackThis. All are great utilities, with their own advantages and disadvantages.
I've been using these types of utilities for many years, and I've become fairly accustomed to what can be removed and what must be kept. New users should proceed cautiously -- only remove what you're certain won't harm your system. Know that you can disable and reenable them quickly (if problems are noted) with many of tools, including Autoruns.
Having a ton of autorun programs has big implications in the enterprise, too. First, if admins don't know of every autorun program installed on the end-user PCs they're managing, there is a security gap. Only allowed and authorized programs should be installed.