This is a utility often used for troubleshooting conflicting programs and resources. You can access it by going to Start - Run, and typing "msconfig". This will open the System Configuration Window, which shows many of the items which start up when the computer boots up. To turn off specific items, choose "Selective Startup". There are a few tabs here, but the most important ones to visit are the Services and Startup tabs. (Note: Depending on your operating system and user rights, you may not see all of these tabs.) Under each tab, you can uncheck items that you may not want running. If, after rebooting, you found that some programs you wanted to load did not load, you can come back into the System Configuration Window and turn them back on.
MSConfig is meant primarily as a troubleshooting tool so that you can find which programs are problematic and then remove them. However, you can use it for an extended period of time with no problems.