please advice, whether that's best done when the virus has made a slow running computer program,
should the anti-virus or in any format?
because I've tried to eliminate the virus on our computer but still very slow,
whereas previously normal and had no problems in terms of speed.

Before you reformat, try the Kaspersky TDSSKiller, which can find hidden files, such as rootkits and master boot record viruses. It is available for free at https://support.kaspersky.com/faq/?qid=208283363 on the web. It will work on Windows XP and newer computers. You will have to unzip the file. Just accept the default configuration that it comes with. It is updated weekly and will tell you when a new version is available.

After you run TDSSKiller, try a scan with Malwarebytes free, which is available at https://www.malwarebytes.org/products/malwarebytes_free on the web. Do a Quick Scan with the defaults that it comes with.

If your computer is still slow, then reformat. Good luck!

Regards,