Please refer to SA's response at this post
MdiChild with different form title[
^] - ignore the question title, just look at SA's comments and links.
I'm directing you there first as he has valid comments regarding MDI forms.
You might also want to look at this codeproject article
Singleton pattern for MDI child forms[
^]
There was a similar question here in codeproject a couple of days ago, so searching for MDI just within this site might also give you some other useful hints