If you still can get into windows and within your computer antivirus software is still there (even if the antivirus is not good), the solution:
1. Prepare to be installed antivirus program, and store them in a CD. Antivirus files will be installed should not be stored in the pendrive, but must be stored on the CD, so as not infected with the virus.
2. Virus busy with conducting a virus scan. So do the virus scan to divert the attention of the virus.
3. When the process is running, install the new antivirus into a PC.
4. If this does not work, install the new operating system, select new installation, select different folders etc ...