UserForm or MsgBox without code interruption?

I have a few functions in my workbook that get called every few minutes by the On.Time event in the background while users are working. Occassionally it needs to report something to the user but I want to do this in such as way that the code continues executing even while the message is displayed. This is because the workbook may be left unattaended but I need the On.Time event to continue to run every n minutes.

Application.StatusBar is not going to be noticeable enough. Is there any way to pop up a message or UserForm part way through these background procedures and have the code continue to run (and even get called again) despite the message showing on screen? I know a message box won't work so didn't even try it. I tried a userform but as soon as it's displayed the code stops until the userform is closed.

I'm thinking I need to call some windows function outside excel but I don't know how to do that. If anyone has a code example that would be great.