I am often asked to look at computers that are slow, have popups, or are otherwise acting funny. I switched to Linux many years ago and then to a Mac and I haven't looked back. That typically means I'm “out of the loop” about Windows. Regardless, this page is an attempt to organize some of the best tools and advice I've seen in dealing with these types of problems.
I currently recommend the free version of Malwarebytes Anti-Malware. It seems to be a good anti-virus solution without the bloat of other tools and it's free!
My brother, who's more of a Windows user than me, suggests using CCleaner to fix speed problems that are not caused by malware.
There are lots of fake anti-virus tools that are actually “trojans”. These are just as dangerous as the virus' that they say they remove. Be vary careful with what you install to try and fix your problem.
One specific example is called “System Fix”. It masquerades as a virus tool but it hides most of your files instead. If you have this specific trojan then there are good instructions on how to remove it at the URL below.
Most computers include a way to restore them to factory defaults. This is the nuclear option, as it will delete all of your data, but it's also the most effective. If you have good backups, it's perfect.
Use a service like DropBox to easily keep everything important to you safe.
When you install DropBox it creates a folder called DropBox under My Documents. Anything you put into that folder will automatically get copied to their own website for safe keeping. It happens rather magically and all you have to remember is to put everything important in your DropBox folder. It's free to get started and then there's a monthly fee if you need more space.
If you install it on two computers it will also put your files on that other computer, giving you even more piece of mind. My wife and I share a DropBox account. As a result, everything important to me is on my computer, her computer, and in the cloud. And I don't have to think about it; it just happens.