Yep. Two things. First, all that antispyware/antivirus comes at a price... slower performance (particularly when you boot up or log on). The scanning of files, etc. eats performance, both in processor power and disk access. The tighter you make your security, the slower your system will run. Period. It's a bit like playing football. If you play without pads, you risk getting hurt. But if you put on 80 pounds of body armor, you'll never be much of a sprinter, no matter how much horsepower you have.
Second, over time, systems get bogged down with "extras"... TSR's (terminate but stay resident programs), browser "helpers", add on's, etc. Windows starts "services" by default that you may never use. Printer drivers or other hardware include programs to "check for updates"... the list goes on and on. All of these "conveniences" come at a price. Lower performance. Do you really need a HP program running in the background to check for updates every day? Or have Adobe Acrobat search several times daily? Pfft... drop these built-in "conveniences", and you drop tons of baggage.
Go to start--run, then type "msconfig". Change from normal startup to selective startup. Then go to the startup tab and uncheck all the crap you don't really want running every frigging moment. Do the same on the services tab if you know what to look for. Then, go to add/remove programs. Uninstall anything you don't actually use.
WinPatrol by BillP is free. It can tell you what's on your system, what's loading on startup, what your active tasks are, etc. Run it, clean your system, then uninstall it (yep... it will run in the background too if you keep it loaded).