I've had a quick look at the Notepad Express application. The Calculator is a bit 'sparce' in functionality. Allowing only multiply, add and subtract and Integer caclulations is a bit of a limitation. Also, when using Calculator, if I try to change the operator (ie I selected + but meant -, the calculation is not re-performed. (I see you used to have a Button to perform the calculation but have removed it from the Form - perhaps you could consider reinstating it)
Being able only to Save As a .rtf file is also a bit of a restriction - I might want to save it as a different filetype (eg .dat, .csv etc).ie plain text as I typed it in, without the rtf tags
Perhaps the name Notepad Express is a bit misleading, if you're going to restrict it to .rtf only perhaps you could re-name it to 'RTFpad Express' which (IMHO) better describes it.
There appears to be a bug in the Save As logic. If I save a file into my Documents Folder called 'test.rtf', and if I have a Folder in the Documents Folder named test, it changes to that Folder and asks me for a filename.(ie it saves to \Documents\Test\test.rtf). If a Folder named test does not exist it saves it in the Documents Folder, as required.
In the code I notice that you call Randomize every time a Random Letter or Number is required. Randomize should only be called once in an Application (normally in the Form_Load event) otherwise the 'randomness' can be compromised.
Also, and this is really for future reference, the Random Letter code can be simplified to:
If you create the Random number in the ASCII range of A to Z (rather than 1 to 26) you can remove all the If / Then and Letter = , statements.
Private Sub RandomLetterToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles RandomLetterToolStripMenuItem.Click
Dim Letter As String
Letter = "NO LETTER"
Dim rndnumber As Random
Dim number As Integer
rndnumber = New Random
number = rndnumber.Next(Asc("A"), Asc("Z"))
Letter = Chr(number)
MsgBox("Your random letter is " & Letter & ".")
Generally nicely written and there's some scope for additional functionality eg perhaps when creating a Random Letter and / or Number rather than issuing a MsgBox, it gets pasted into the current cursor location. Also, you could ask the user how many Random Letters or Numbers to create and, perhaps in the case of Numbers what the range should be; in the case of letters, whether they should be upper/ lower / mixed case.
(&, of course, there's plenty of scope for expanding the Calculator functionality)
I'm sure you'll have some fun !