I installed linux mint12 recently
What does recently mean, timewise?
Have you used the Software Manager on Mint12 successfully? or has it no worked since install?
Does this problem occur when trying to install specific software? or when you try to install any software with the Manager?
When you click on Software Manager in the Mint menu, do you get the Software Manager window?
Are you able to click on a category and access the category?
When you click on an Application to highlight it, do you get a window which indicates whether it is installed?
If so, do you see and Install button on the right?
Where in this process does it "not start the task"?
A little more info would help.
Does Synaptic work?