If it says that it is already on PC then you probably havent completely uninstalled it.
1)I presume start , control panel , add/remove programs , remove has been tried ? If not then try this.
2)Start , all programs , AVG Free Edition , highlight and click Uninstall AVG Free.
3)It could be that for what ever reason it hasnt completely uninstalled so we could try to find any dead links to it with this tool called appropriately enough "Find dead links" click here .
4)You could also try an uninstall tool My Uninstall from here click here
A few things to try , hope this is of help to you !