Website Hacked

I noticed yesterday that Eset Security was picking up a virus coming from a few of the websites i have.

When i went to try and figure out how this happened, i noticed that on a few of the php pages i have the filesize was bigger than it should be. When i opened the php file i found that code had been added to the top of the php file which seems to redirect the page to another dodgy site.

Basically malcious code is added to php files and html files;

I havnt found out how this happened or how to fix it, more info to come.

Update: 28 Dec
It seems my entire website has been infected, i had thought it was just php pages that got infected but its ordinary html files as well. A college assingment i did a year a go has been infected, well the index.html page has been infected. Again i have no idea how, its the same virus.

Ive scanned my sites with a online scanner which does a good job of telling if your site is infected;
however how to fix it is proving to be difficult. My hosting company havnt replied back to me yet so im searching for a solution.