It IS possible that too many applications running in the background is using too much system resources; if your computer seems to take a long time to boot or to shut down, this would also be caused by unneeded applications launching and shutting down.
Since you have Spybot, you can easily use its Startup Program Tool
to review and manage your startups; launch Spypot, and look for Mode
at the top of the screen, click on it and choose Advanced
to get the tools panel on the left side of the screen, expand tools and look for system startup
Generally, the applications in green are OK, yellow are unknown, and red are malicious in nature. Looking at the command line column on the list should give you some indication of the program that can, in addition, suggest to you whether you need it or not. For example, many media players will by default run in the background; if you do not use these a lot during your session, you can use the Tool to stop them by unchecking the box on the left of the line.
Do NOT uncheck anything you are not complete certain about. Instead, us BC's Startup Program database
to research what each does before making a decision.
See also the pinned topic, Slow Computer Checklist
for other tips to a faster computer:http://www.bleepingcomputer.com/forums/t/44689/slow-computer/
Following those steps may or may not resolve your problem, but they will eliminate many possibilities when you post back if they don't solve your problem.