istockphoto LC stock photo company has viruses on their files ontereo, Ontario

I bought rights to some stock photos from istockphoto.  The moment I paid for them and downloaded them I started having problems with my computer.  It changed my homepage to yahoo.  Ever since I downloaded them, every time I start my computer this popup window comes that says Registry Booster.  It slows my computer considerably.  I have to wait at least 5 minute for it to do its thing before I can do anything.  Also I downloaded the pictures to my hard drive okay, but when I try to use them I get another popup from another company that says I must subscribe to their newsletter and pay $19 before I can use the picture.  It was and is a waste of my time and money. Please don't be another one of their victims.  They deny responsibility and won't refund my money.