Do not get zone alarm it bogs down your system a great deal. I have avast myself and whats seems to cause any slowness I receive is when its downloading daily updates.
Few suggestions though (follow in order)
Scan the system with spybot.
Scan the system with some kind of anti-malware software. (I used a-squared)
Scan the system with Avast
Get a copy of SpeeDefrag and run it.
http://www.download.com/SpeeDefrag/3000-2086_4-10491590.html (It reboots your pc and runs only defrag.exe, defrags your system in seconds)
You may also want to try JKDefrag
http://www.kessels.com/JkDefrag/
Hope that helps.