Use Ccleaner!! often this tools helps you clean your instalations, temporary files, and disable startup programs
Clean up your registry, and temporary files often with CCleaner (Crap Cleaner) when installing a program windows records info about it on the registry and very often this info remains after you unistall it so they are making things slower but aren't being used at all!! http://www.filehippo.com/download_ccleaner/
Disable the most start up programs you can this will reduce the RAM usage on idle so you can use more of it when you really need it
Do you really use everything you have installed??