This question is coming back often, and the suggested steps are very often the same. In an objective to concentrate useful information in one place, here is a community wiki about it. I expect our best minds to participate in it, so we can have a reference of knowledge, for something like this.

What to do if it seems that my computer is infected by a virus, or a malware on a Windows system?

• What are the symptoms from an infection?
• What are the steps to follow after noticing an infection?
• How can I get rid of it?

As it is community wiki, feel free to edit this question to improve it as well.