I run it every now and again with o&o
makes sure ALL files are defragged properly, I always do this first too..
ensure system restore is off
-delete all restore points & volume shadow copies
run ccleaner, ticking as many options as I can without it being annoying (removing firefox saved passwords etc)
remove any non required folders on C:\
like C:\Nvidia, C:\ATI, C:\INTEL etc etc..