which software is best?
There is no universal "best" answer for which anti-virus software to use. Asking four different people with experience in the IT field will likely give you for different, equally valid answers. Everyone has a favorite anti-virus program, and reasons for choosing that particular piece of software. I'll offer up some guidelines that, once you understand them, should be common sense.
1. Don't pay extra for protection
There are several well-respected anti-virus packages that are 100% free to use as long as you are not using them to operate a business. Even as a business, some free-for-consumers software remains free for fully licensed versions. There are still some consumer-level software packages that operate on a subscription (monthly or annual payments) model. I have yet to find any compelling evidence that any for-pay software is any better at detection or avoiding false positives than the better free alternatives.
Besides the obvious financial reasons, there is a risk in running for-pay software. Once a subscription expires, the icon usually remains. This leads many users to falsely assume they still have virus protection running. What is left running (often a reminder to send more money to the software manufacturer) can also interfere with other valid anti-virus scanners.
2. Running multiple programs does NOT offer more protection
In all but a few rare cases, anit-virus software scans for the same things, the same way, using the same resources. In most instances, having more than one anti-virus scanner running will actually reduce the level of protection. Conflicting anti-virus software can result in false positives as the other scanner works, or worse, failing to detect malware that would otherwise be detected.
3. Don't even touch "Registry Cleaners"
The registry is where Windows store all of it's instructions on how to run programs, what programs get access to what resources, and which programs are used when you open different files. It is the core internal set of instructions for Windows, and changes every time you install a piece of software, update something, or even change your desktop settings.
There are plenty of "registry cleaners" to be found online, all purporting to fix any and every ill your particular Windows installation may have. The problem with this approach is that software designers have no idea what is actually in your registry when these programs are written. The best they can achieve is a generic "should work on most computers" approach that can break more than it fixes if your registry doesn't look like the software expects it to.
Changing settings in the registry can instantly crash Windows, and make it impossible to ever boot again. The only way to correct such a mistake is to re-install Windows and start over. The registry is a very powerful, very useful tool that should only be altered if you know exactly what you are doing and how it will affect how Windows operates.