Unless VB.Net has a mouse off event (which it might, I don't know), The answer is "there isn't one". I know VB6 doesn't have one. The only way to find out if the mouse has gone off the form is to use an api call. You need to use the getcursorpos api, and pass it the POINTAPI Type. This will effectively give you the current X Y coordinates of the mouse.
Then, you get the current X, Y coordinates of your form (maybe using getwindowrect, with type RECT), and then compare to see if the mouse falls within those boundaries. If it is within those boundaries, the mouse is on the form. If it's not, then it isn't. So you could use a timer control, and check every millisecond where the mouse is. As long as it's on your form, set a variable to true. When it leaves your form, set it false, and execute some kind of "off form" code. Another option, is to set a global hook on the mouse, and whenever you recieve a WM_MOUSEMOVE, check it's coordinates, compare, and execute.
As for the checksum, I found This neat information:
http://www.vbaccelerator.com/home/VB...32/article.asp 
Unsolved 
