I'm not a big fan of Microsoft Windows but my bank is. They make deposits easy and scanning checks on my old Canon LIDE60 is flawless. I recently re-provisioned a Dell Optiplex 780 (surplus ~$50) with Windows 7. No hurry for Windows 10. Microsoft provides OS updates and anti-virus and anti-malware software which is free and adequate for the task. I see no advantage in any of the paid anti-virus and anti-malware software. For non-Microsoft software I head to http://filehippo.com/.
The first thing I grab is http://www.filehippo.com/download_app_manager/. This will help me keep non-Microsoft software up-to-date. I get the vast majority of my software through FileHippo. I almost never download trial ware except for one-time use of anti-malware software that does not have a "free" version. I set up the FileHippo App Manager for daily scans for updates.
The next program I grab is http://filehippo.com/download_ccleaner/ I run CCleaner about weekly and before any other scanners to reduce the amount of trash scanned. CCleaner has a registry cleaner and an efficient Un-installer. If you rarely use software and can re-install it for free, get rid of it. It also has a "StartUp" editor. Performance issues are most often the result of inadequate memory (2GB is a functional minimum these days, 32 bit Windows doesn't know what to do with more than 4GB). My surplus 4GB Dell cost less than an upgrade for older boxes so invest wisely.
The next program to grab is http://filehippo.com/download_malwarebytes_anti_malware/. It takes a long time to run so I reduce the time by uninstalling fluff first, then CCleaner. You must let it run to completion rot it to actually remove stuff. Sometimes even over night. Better to run in "normal" mode than safe mode but if your machine is in bad shape do it in safe mode (tap F8 on restart and choose safe mode with networking). Run it again if it detects anything and quit only after a clean run. Do this weekly until you machine seems to come up clean often, then run it monthly. Don't wait until life gets bad or you may have to engage in heroics to get control.