Problem in EventLog in Forms Authenticaiton

Dear,
If you try to use the below lines of Code....it will not if your authentication mode is Forms.... but it's work fine if your authentication mode is Windows..

EventLog aLog = new EventLog();
aLog.Log = "System";
aLog.MachineName = ".";
LogGrid.DataSource=aLog.Entries;
LogGrid.DataBind();

i tried alot... to solve it..... but nothing solve my problem... even the class doesn't support to take Network credintial object to set it at runtime...like any other classes in .Net ................... :(

Regards,
Moustafa arafa