Using iMac as an external display

Hi everyone

I'm looking for a little help and will apologise in advance for my total lack of knowledge.

I'm a recent'ish mac convert and have the lot; 27" iMac, macbook air, ipad, iPhone.

My little problem is this (and I've searched around the internet and nothing seems to make any sense to me); I work from home one day a week and am desperate to try and get my iMac (2010 model) to work as an external display to my IBM Lenovo work laptop. I've bought a mini display port to VGA cable today and tried to hook it up, but I when I toggle on the display it goes blue for a few seconds and then flips back to the normal iMac display.

Does anyone know if this is even possible ?

I could have probably saved some money by just going out and buying a seperate LED monitor for my work laptop, but I hate being beaten by this sort of stuff.

Any help (in words of one syllable) would be really appreciated.

Thanks