Do a Clean Install Windows 7
Before going for the clean install, better you optimize your hard drive for renstallation as you suspect a virus . Follow the tutorial SSD / HDD : Optimize for Windows Reinstallation
This will show how to remove all existing data from a Solid State Drive (SSD) or a Hard Disk Drive (HDD), it'll work for both, then how to create a partition and then format that partition before install begins so that you have a clean drive to start the installation with.
BTW, there are many causes for a computer getting slow other than viruses . You may check how many programs are starting up alongwith windows and running in the background . Press windows key ard "r" simultaneously to get the run window . Type msconfig there .... and hit enter . Look at the startup tab of the resulting window .... if there are some unnecessary elements in the startup folder, you may disable those .
For the virus issue, download , install and upgrade Malwarebytes' Anti-Malware
and run it .