Watch out for Tech Support Scams
These scams typically start as a phone call or fake webpage that you may have clicked in a search result. In the phone version the caller will claim to be from Microsoft or some other tech company and they will tell you that your computer is infected. They will typically try and get access to your computer to help fix the issue. After running you through several steps to show you the infection they try to sell you software to fix the problem. The webpage variant will popup messages indicating your computer is infected and that you need to call Microsoft or some other company at the listed number to fix it.
These are all scams, Microsoft will not contact you to alert you of an infection. Just simply hang up the phone or power off your PC. Be cautious of popups and check boxes offering to disable future alerts. They can be very sneaky with images that look like legitimate messages.
"Refund" scams are similar and involve a call from someone to followup on a previous service or virus removal and offering a refund. They are only after your bank account numbers so just hang up.
Your best defense against this is to backup your data and have a good security plan. In a business environment we recommend having protection from several different angles. Antivirus software alone just isn't enough.