Dilemma updating UI from seperate thread

I have encountered a problem that I am having trouble solving. I have one main UI. From this UI you can bring up a new non-modal Windows form that displays some other data. However, each time a new form is brought up, it is done so in a new thread. What I want to do, is have these new forms (in their new thread) make a call to a public method in my main UI's class. This method(funciton ) accepts a string argument (and returns a string as well), and basically adds the string that it is passed as an additional menu item on the "Window" menu drop down of the main menu bar. Then when I close one of the spawned forms, I want to make a call to another public method to remove it's menu item from the "Window" drop down list.

What I have tried so far, is adding this code to the Form_Loading event of the new form (remember though that this is on a seperate thread than my main UI -- the class which contains the public method I am calling).


myNameIs = myParentForm.Ad dWindowItem("Na meOfThisForm");


The AddWindowItem method is a public method on my main UI class. It's code is as follows:

public string AddWindowItem(s tring windowName)
{
bool isLikeExisting = true;
int i = 1;
string cachedName = windowName;
string originalName = windowName;
while (isLikeExisting )
{
foreach (MenuItem mnuTest in mnuWindow.MenuI tems)
{
i++;
if (windowName == mnuTest.Text)
{
windowName = windowName + i;
}
}
if (windowName == cachedName)
{
isLikeExisting = false;
}
else
{
cachedName = windowName;
windowName = originalName;
}
}
mnuWindow.MenuI tems.Add(window Name);
return windowName;
}


Then on the closing event of my "spawned off in it's own thread" subform I do:

(myParentForm is an object instantiated as the main UI class that initiated this new thread/form)
myParentForm.Re moveWindowItem( myNameIs);


The RemoveWindowIte m public method which resides in the main UI class on the main thread is as follows:

public void RemoveWindowIte m(string myItemName)
{
int indexToRemove = 0;
foreach (MenuItem mnuTest in mnuWindow.MenuI tems)
{
if (mnuTest.Text == myItemName)
{
indexToRemove = mnuTest.Index;
}
}
mnuWindow.MenuI tems.RemoveAt(i ndexToRemove);
}



I'm sorry if this is very confusing. If anyone knows a way to accomplish what I'm trying to do, or can help in any way, I would greatly appreciate it. Thanks!

Some additional info: Right now it will add the new menu item, and successfully remove it, but for 1 newly opened non-modal sub form only. If I try to open a second instance of the sub-form, nothing happens.. the form doesn't even open. But then when I close the 1st one I had opened, the second one I tried to open appears.