I know there are still some people using XP and they have probably come across the Fake AV virus. It is really annoying and seems to target XP machines. Here is how to fix it.
1. Download Malwarebytes free version (free malwarebytes)
2. put malwarebytes on a thumb drive
3.Reboot your pc and press F8 a few times before the XP splash screen comes up so you can get into Safe Mode. Boot into Safe Mode with Networking. (so malwarebytes can update it's database)
4. Install Malwarebytes and perform a full scan.
5. Remove all found threats.
6. Reboot computer normally.
Usually this is the end of it. There are some versions of the virus that route your internet traffic through a proxy server. You will know this is happening if you still cannot access the internet. To fix this open Internet Explorer and go to Tools>Internet Options, click on the Connections tab and hit the LAN Settings button. Uncheck the box that says "Use proxy server."
That is it. I've run into this virus (for you nerds, I know it's actually a trojan)at least 5 times that I can remember, not counting yesterday. Anyway, hope this helps.
Upgrade to Windows 7 when you get the chance. It's worth it.