You can download a free anti-virus. Search AVG, download it, and use it. It's better than having no protection.
I would suggest purchasing Norton Anti-Virus for $60 or whatever. It's worth it to have a computer that runs smoothly.
Another solution would be to use a system restore command. This will revert all of your files, folders, programs, etc., back to what they were at an earlier date. This would eliminate any changes in the registry, and whatever is on your computer. The drawback is that you will also lose all of your information that has been added since the last system restore point.