The new Windows 7 beta release just like Vista preview releases before it incorporate some additional UI elements meant for testers to submit feedback back to Microsoft.
Now considering you may be running this Beta 1 release as your main OS for some reason (I admit it, I have given in to the temptation many times before), one of those elements in particular can become annoying and obtrusive to your otherwise neat configured desktop.
Read the complete post at the TechSpot Blog.
As a side note, I have also posted a little trick that is being used to get you a Windows 7 Beta key despite of the official site being shut down. It's not working for everybody but might be worth a try at least.