Trojan Win32.Medfos.A is a Trojan Horse that is programmed to redirect the opened web browser (Internet Explorer or Mozilla Firefox) to pay per click websites, e.g. greatfilehosting.com and midifilehosting.com. The other feature, characteristic to this Trojan as well as its other version Medfos.B are lots of pop-up advertisements that interrupt work with an infected computer.
Win32.Medfos.A can be infiltrated into a computer using system vulnerabilities. Cyber criminals use common methods for its distribution, such as sending spam emails having corrupted attachments, adding malicious code to downloads, tricking computer users on social networks by making them click on a link with a virus, etc.
After Medfos.A gets inside a computer, it modifies the system registry in order to be launched every time Windows is started. The Trojan earns money for its creators by redirecting infected computer users to pay per click sites. Every time you click on such a site, you generate money for Medfos.A owners.
There is no need to tell that this badware should not be ignored and must be removed as soon as any of its activity is noticed. Antimalware tools like Spyhunter can detect it and remove all its related files automatically. For the future check if your firewall is enabled, make sure all of the computer programs are up to date. This will decrease the risk of getting infected again. Also be cautious when opening attachments and clicking on the links. Do that only when the source is reliable.