Here's a few things I do to keep my computers operating efficiently. I am no expert, but I rarely have issues with computers running slowly or not at all.
1. No Internet Explorer, ever!
2. Accept updates.
3. Use free antivirus software and keep your firewall on. I use free AVG.
4. Avoid agreeing to extras when downloading free stuff. Sometimes useful tools are included but most are a nuisance.
5. Spend less than $50 a year on a software subscription that keeps your computer running efficiently.