Damn! I had that exact thing pop up last week. After I figured out that it was seemingly a Java Applet and I ran McAfee (and it found nothing!) I just let it sit and waited to see what it might do. Ultimately, left on it's own it popped up 4 windows (touting that sale) and sat there. At one point I restarted it and kept it disconnected from the internet and the web page still appeared! That was the key that it was a Java thing. Since this was a work laptop that has heavy encryption, and - I'd recently had a HD crash so everything was newly installed anyway there was no damage it could do really - but I'm not sure I'd let it sit like that without first running a virus detection program.

Long story short, McAfee found nothing, I ultimately shut it down and used another computer until I could get back into town, and 3 days later when the tech guys came to see it, it was all gone! No sign of it since. No clue really as to what is was, but the consensus is that that company popped the Applet on it, programmed it to annoy the user for a day or two, and goes away naturally after a day or two.

The key for me would be to immediately run a virus detection program in case it's something more destructive and just a temporary annoyance.