Backed up files are infected
If the computer is infected with a virus at the time you backed up your files, your files can be infected. It is recommended to scan your files before you put them back on the computer. My recommendation is to scan it using two different anti-virus programs. Make sure they are fully updated before you scan.
If you do not make sure they are clean from viruses, you can put the virus back on the computer after you have done a reinstall.