say your combo box is called cboForms, go to the Afterupdate event of cboForms and paste in the following between the Private sub and end sub lines:
Dim strfrm As String
strfrm = Me!cboForms.Value
If IsNull(Me!cboForms) Then
MsgBox "Choose a form first!"
Exit Sub
Else
DoCmd.OpenForm strfrm
End If
See if that does the trick.
PS, Make sure your combo box is bound to the column holding the form name