I would recommend you to use a good antivirus software which is widely used and effective such as AVG, Norton, Search and Destroy etc. Keep antivrus updated. Try updating windows especially security related patches. 

You can also use a registry cleaner along with antivirus, It will help increasing your PC speed and can clean remains of spywares, viruses . Good registry cleaners are CCleaner , Registry Mechanic but my favourite is RegInOut http://www.reginout.com