How can I control (start/stop) applications and services that start up after reboot? I have a Mac that is slow when using the internet, but other PCs on the same connection aren't as slow.

I suspect there are too many AV scanners running concurrently, or there is some process that isn't installed by default slowing the whole thing down.

Question

Where can I find all possible applications that will be automatically started when the machine turns on?  (and how do I shut them off)

This should include


Applications on the menu bar on the bottom 
Services (like Antivirus) 
Menu-bar applications
Applications and Menu Bar items

Most of these can be easily configured in:
System Preferences &#x2192; Users&amp;Groups &#x2192; Login Items

LaunchAgents and LaunchDaemons

Some applications are cannot be configured in the System Preferences (e.g. Sophos AV Scanner, HandsOff Firewall,...) It's best to configure them in their own application to assure that everything runs as designed and intended.

You can look for related LaunchAgents and LaunchDaemons in:

~/Library/LaunchAgents               ~/Library/LaunchDaemons
/Library/LaunchAgents                /Library/LaunchDaemons
/System/Library/LaunchAgents         /System/Library/LaunchDaemons


Startup Items

After I installed the multi-boot utility rEFIt, it copied a file rEFItBlesser which is run at startup.
Items that run at startup can be found in the following locations:

~/Library/Startup Items
/Library/Startup Items