Assuming that you have a valid Windows 7 Product Key, either affixed to the outside of the computer somewhere or contained within some documentation that came with the system and that the computer still boots into normal mode, you should be able to perform what's called an in place upgrade / repair.
You can download and burn
the installation media from here.
Be sure to get the version that matches the product key you have.
Brief instructions here.... it's pretty simple, burn the installation media from the ISO that you download, boot your system, insert the media and choose upgrade.
Although this is generally a non-destructive process, I would still warn.......
1. Backup everything you consider important.
and
2. If the machine is or was truly infected then this will not likely resolve the issue although it may provide a platform from which you can address any infections properly.
It can take a while so be prepared to be patient.