1.Disable all the startup programs, you don't really need them running all the time.
2. Observe the Task Manager and see what's hogging most of the resources and scrutinize unnecessary active software and remove it if necessary(DO A THOROUGH WEB SEARCH FIRST BEFORE UNINSTALLING ANYTHING).
3.Remove gunk files with Bleachbit.
4. Install firewalls like TinyWall and Peerblock and add only the specific rules you need, don't leave your PC port wide open.
5. Install only the stable version of the software, some unstable software are resource hog and can cause memory leak.
6. Make sure everything is up to date.
7. Make a complete backup of your system and do a system reset.
And also don't forget to scan your system for viruses and malwares.
Practice a healthy computing habit and you won't have to worry about anything even running without antivirus software and you'll be fine.