How to tell if you have a virus — and what to do next

Most "I think I have a virus" moments turn out to be something else: a full disk, an old machine, a bad update. But some symptoms are worth taking seriously, especially when several appear together or something changes that you didn't change yourself.

Common signs of malware on a computer

The US Federal Trade Commission's list of warning signs applies to any computer:

• Your computer slows down, freezes or crashes.
• Your browser's home page changes, or it keeps taking you to sites you didn't mean to visit.
• New toolbars or add-ons appear that you didn't install.
• You see lots of pop-up ads.
• You get repeated error messages from the operating system.
• Tools like Task Manager or Activity Monitor won't open.
• Friends receive emails or social posts from you that you didn't send.

Signs on a phone

• Battery drain or data use that you can't explain.
• Apps you don't remember installing.
• Pop-ups appearing outside the apps you're using.
• Contacts receiving messages you didn't send.

Any one of these can have an innocent cause. Look for patterns — and for changes you didn't make.

What to do if you think you're infected

1. Stop logging in to accounts that involve passwords or sensitive information, such as banking and email.
2. Update your security software — or install it from a reputable vendor's official site — and run a full scan.
3. Once the device is clean, change your passwords and turn on two-factor authentication. If you're unsure, change them from another device you trust.
4. Check your bank and card statements for transactions you don't recognise.
5. If problems continue, contact the device maker's support or a technician you trust.
6. If you've lost money or your device was accessed, report it to ASD's Australian Cyber Security Centre at cyber.gov.au or on 1300 CYBER1 (1300 292 371), and report scams to Scamwatch.

The fake virus warning is often the real threat

Many scams start with a pop-up that looks like it's from a well-known company, claims your device is infected and gives a phone number to call. Real security warnings never ask you to call a number. Close the browser tab — force-quit the browser if you have to — and never give a caller remote access to your computer.

How to avoid malware in the first place

• Don't click links in unexpected emails or messages.
• Don't download software from ads, and avoid unfamiliar sites offering free downloads or peer-to-peer file sharing.
• Keep your operating system and security software updated, with automatic scanning on.
• Scan USB drives and other external devices before opening files from them.

A slow computer is usually just a slow computer. A pop-up telling you to call a number is never your antivirus.