Recovering from a virus or other malware
Your aim is to update your antivirus software then isolate your computer so that the malware doesn’t spread.
On Windows 10, run the Malicious software removal tool: https://support.microsoft.com/ en-us/ help/ 4026667/ windows-10-how-to-remove-malware-or-viruses [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] .
If you have suffered a ransomware attack you may be able to use information provided by https://www.nomoreransom.org/ to recover your data. This is a scheme set up by Europol, the Netherlands Police, McAfee and Kaspersky to analyse ransomware and identify the decryption keys to recover data (see https://www.bbc.co.uk/news/technology-49096991).
In a worst case scenario, you may need to reformat your hard drive, reinstall your operating system and reinstall your keys. You will then need to reinstall any programs you use and then your data from your secure backup files. (You have got them all safe haven’t you?)
Note that the reason for a slow running, old computer can be a build up of dust in vents, fans and internal surfaces so that the processor slows down to avoid overheating.
Once you have completed these steps, spend a few minutes thinking about how the malware might have got on to your computer. Did you visit a suspicious website, download a suspicious program or simply click on an attachment in an email message? These are common ways to receive malware, so think about what you can do differently to prevent it happening again.