You're missing a lot of detail in your question, but I'll assume you're at least running one of the last 2-3 major releases of Windows. I'll also assume you've correctly identified viruses as the cause of your trouble, rather than hard drive or memory issues.
You're going to need to download and Install a program called MalwareBytes. You may need to use another computer to download the software, then transfer it to the machine you're going to work on with a USB stick/thumb drive (or equivalent).
If you can't boot up into Windows normally, try booting into Safe Mode with Networking.
Once installed, launch MalwareBytes and allow it to scan your PC. I recommend allowing it to remove pretty much anything it finds and flags for your attention. If it does find anything, re-run the scan again after you've allowed it to remove the infections. Repeat until your scan comes up clean. After that, reboot the machine (into normal Windows mode, as opposed to safe mode) and see if everything is better.