AE isn't an editor, and playback is sporadic generally unless you do a RAM preview(press 0 on the number pad).
Info and Links on RAM preview.
As far as what can be deleted, you only posted your system, we don't know what you are doing, what files you are using...
As you are new to AE Todd Kopriva of Adobe has a great list of references for getting started:
Getting started with After Effects (CS4, CS5, CS5.5, & CS6)
Along with the links to Andrew Davis and the VideoCopilot series, I strongly recommend the book Creating Motion Graphics by Chris and Trish Meyers.
Johnny Cuevas, Editor
Thinkck.com
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.