Since you are posting
this problem on a Mac thread, might I suggest that the solution is a Mac?
In all seriousness, your problem may well have originated with malware infecting your Windows machine via Explorer. Whether or not this is the case, one of the top recommendations for security
with Windows is to stop using Explorer - it is just too great a security
risk. Use either Google's Chrome or Mozilla's Firefox for your browser, and particularly with Firefox consider adding security extensions.
Personally, I use a Mac with ClamXav anti-malware, and Firefox with a rather long list of extensions. With ClamWin and Firefox with appropriate extensions Windows security can be made acceptable.