C:\Windows\Installer is where Windows Installer stores cached copies of the setup packages (.msi) and setup patches (.msp) used for your currently installed programs. These files are required if you want to update, modify, or uninstall a program on your computer. Do not delete them blindly.
Sometimes you can end up with orphaned .msi or .msp files in this directory that do not get cleaned up appropriately. This typically happens when an installation fails. These can be removed with a tool called Msizap, but be aware it is not supported by Microsoft anymore.
Msizap can only be downloaded as a part of the Windows SDK Components for Windows Installer Developers that gets installed with the Microsoft Windows SDK. Once installed, you can remove orphaned installers from C:\Windows\Installer with the following command:
"%PROGRAMFILES%\Windows Installer Cleanup\msizap.exe" !g
Here is the usage information for more details: http://msdn.microsoft.com/en-us/library/windows/desktop/aa370523%28v=vs.85%29.aspx
I would honestly not worry about 10gb of installers sitting in there unless you are critical on hard drive space. It is more likely a sign that it's time to reinstall Windows again to get your computer running as fast as it did on the day you bought it. 10gb is pretty typical for a 2+yr old install of Windows.