the best way to prevent all virus and spyware is not to connect your computer to any other computers or to the internet. there are many tools that you can find with google to help you remove malware and spyware. I also encourage you to check out Ubuntu or Fedora linux. I switched to linux back in 2008 and since then have never had any malware or virus.
Often a virus prevents you from installing an antivirus. You should always clean your machine first, I use combofix and malwarebytes in that order.
Try downloading combofix.exe to your desktop.
Stop any active anti-viruses you have running.
after it is finished run malwarebytes
install antivirus of your choice.
If you can't download combofix and malwarebytes and a virus erases them from your flash when you try to copy them over, then download a copy on linux, Lubuntu is easy, and burn it to a dvd. You can now start your computer from the dvd and then transfer the files to the infected machine's desktop from your flash disk. Yes, you can navigate around in windows with your live linux.
Like the earlier poster, been on ubuntu for a decade now, never looked back, but I have to fix other people's machines.