Strange screen freeze Lion 7.4 / MBA (2011)

Hi all,

I am a newbie on Mac, so bear with me please.

I keep getting my screen frozen (display off, key light remains on) every time my cursor hits the top left screen. And upon pressing any key / pad, it reawakes to request my password. Once done, it goes back to whatever I was doing before.

Is this normal? I suspect it is not, but worried if this is something more than a bug. Any advice would be most appreciated.

Oh, and the only programs loaded are, Office 2011 Personal, and an anti-virus software (Indigo).

Thanks!