I would recommend downloading hijackthis and posting the results on the hijackthis thread. Chances are you have some of the more obnoxious apps running, like yahoo messenger, which always make your system slower on startup and shutdown. A good system cleanup, using something like ccleaner, then defragging your registry and your hdd will also help. Adding memory also speeds things up and is the easiest of all the upgrades to any system. Adding an AV program and/or anti-spyware will actually make it slower if the problem isn't viruses or spyware. Most of the time a gradual slowdown is indicative of a bloated, fragmented registry, and defragmenting your HDD does absolutely nothing to change it.