Any link you clicked could be the culprit.
You don't need to download anything, although an unknown .exe or .com or .bat program could do it too.

I'd personally recommend (after the format) running Firefox with the No Script addon.
If you don't have an anti-virus type program, get one.
I personally like Kaperski, but Norton or McAffee are great too.

Sorry this happened to you.
The people who do this shit are the scum of the earth.
But then people buy gold and such, which keeps the scum in business too.