What is this?

[manchild]

It started appearing and disappearing while FF/youtube and MSN LIVE were active. I'm not sure which one is related to it, although MSN was going wild using over 80% of CPU on idle.

The white rectangular box with IE logo in smaller yellow box, what is that?

[mx_tifoso]

Drop down advertising?

You have some of the craziest threads.

[Pandamasque]

White is underrated. It should be advertised more.

[andrew]

Looks like an advert that haas frozen either that or it is something to do with Internet Explorer going by the logo.

[JohnsonsEvilTwin]

That may be the first signs of you needing a new computer.
Software conflict.

Or reload windows

[Ciro Pabón]

Do you use Messenger Plus? Check the list of processes in Task Manager. If you do, maybe that's adware in your PC.

[manchild]

Yep, it is Microsoft popup add for IE8, that appears when MSN is on. Never seen such thing before.