This seems to be a possible virus infection.
First of all, give your antivirus a test by downloading the EICAR antivirus Test File *www.eicar.org/anti_virus_test_file.htm. This is a simple test file, which is intended to be detected as a virus by Antivirus Softwares.
May I suggest Avast Antivirus (Free Version). Have been using it for 4 years now and feel like it's very good. I use it along with the Microsoft Security Essentials for a bit added protection, but feel Avast is enough. Will not comment on others I've not used except McAfee, Norton and AVG Free (in my early days of computing), which are resource hogs. Avast has both free as well as paid versions, and is not very memory intensive, moreover you get daily updates if you're connected with the Internet.
Avast has a useful feature "Boot Time Scan", which will prompt you to run an antivirus scan during boot, which helps in detecting and deleting anti viruses before they run in the windows environment.