I've been having this issue myself for a less than a week and have been keeping an eye on any info that pops up on the internet about it.
It appears to be a Windows Live Messenger problem, and from my understanding, it's probably part of the advertising the program has showing while it's running. I reported the problem to the Messenger people earlier today and am hoping they figure out their problem.
The best info I've found so far is at this Yahoo Answers link, the very first info I could find was this.
This issue is annoying me to no end.
For the last 4 days all of a sudden, I am getting a box popping up on my computer that wants me to download/save/cancel a file called: 234x60_3f.swf from some place called crtv.mate1.com.
I have never been and don't go to any of those kind of sites if that mate thing has anything to do with dating or hooking up or whatever. I am the only user of this computer so it was not done by anyone else either.
I am using Windows 7 and I am thinking it is coming up when I am using Windows Live Messenger, because when the box pops up, you know how you can just mouse over a program in your task bar I do that on the msngr prog, and 3 windows come up to choose from.. (the main window, the open chat with my mom, and the download box) .
I have checked and rechecked my settings on messenger and it has always been set to private. I don't accept anyone I don't know to chat. In fact I only chat with 3 people ever. My mom, brother and sister. So I cannot figure out why I can't this to stop asking me to download it. I just keep hitting cancel and it goes away for only a little while.
The other day I was gone all day and came back to 13 boxes i had to cancel on my screen.
I use Firexfox as my browser, I have virus protection, script protection, adblocking to name a few for addons.