Arial  Suggested by abo 


Microsoft Sans Serif  Suggested by abo 

Its onboard Arial.

not sure...

o.k. I took a closer look, upper ends of small t looks different. Then how about Microsoft Sans Serif? This used to be the UI default font for dialogues.

Though it might look somewhat different because in the beginning MS Sans Serif was a bitmap font.
http://en.wikipedia.org/wiki/MS_Sans_Serif

at least the "a" is different.

as you can see, it's a fake windows alert, so the font is probably not a MS font

I'll use both thanks for your help you guys.

It's Arial. Main text: 18 px (w/o anti-aliasing)