IT help needed: What the hell is this...

[General Accident]

thing in my XP taskbar. I've never seen it before and it only appeared after I fitted a new formatted HDD to my old laptop and put a fresh install of XP Home on it. Task manager says its not an application, and when I click it it does change colour (light blue/dark blue etc) but nothing happens. It's freaking me out a bit.

Any help would be much appreciated.