I said I'd give you an update if and when I heard from ImageShack, see my post here. Well I heard back from them today.
Some advertisements contain a X that looks like it can close the window. However, this is not the case. In order to close advertisements (anywhere, not just on ImageShack), you must click the X in the VERY UPPER RIGHT of the image advertisement. If you do that, you should have no problems with popups or popunders anywhere on the internet.
ImageShack strives to prevent deceptive advertisements from appearing, but we cannot always control our advertising networks, although we follow up on every issue that we encounter.
Let me know if you have any questions.
Hmmm... well first of all I will say thank you to ImageShack for getting back to me. However, I'm a little worried by the statement that they cannot always control their advertising networks.... why on earth not?? In my opinion, any business that serves up this type of advertising should know exactly who is advertising with them and the methods that they use to procure interest in their product. Or perhaps it is just the dollars that they are interested in, you know, rake in the revenue and wait until someone complains!
Incidentally, when you are hit by a rogue advertisement or site, clicking the X in the upper right hand corner of the image doesn't do any good (I do know how to close windows you know!!) and I'm sure there are plenty of people out there on the internet who will confirm that. A safer and more effective way of closing a window when you are in this situation is to press Alt + F4 this will close the active window without a mouse click.
Finally, I've been back to the images that were serving up the WinAntivirus download and they now seem to be clear. I don't know whether ImageShack have actually got rid of these ads or not. As they didn't confirm or deny this particular problem in their reply to me. I don't think I will be recommending their service to my friends though, I can't take the chance that this sort of thing won't happen again in the future.