I'm guessing that someone wrote a macro that inserts a new sheet and that they
used ctrl-N as the shortcut key.
You could verify this by starting excel in safe mode:
close excel
windows start button|Run
excel /safe
And testing it out.
If that works ok, you'll have to find the macro that has that macro (and
shortcut key).
I'd close excel and reopen it normally.
test out ctrl-N
if it adds a new worksheet, then I'd hit alt-F11 to see the VBE.
Hit ctrl-r to see the project explorer.
Look at the projects/files listed (like windows explorer) in that window.
Try to find the suspects.
(Could it be your own Personal.xls???)
When you do find it, you'll have to find out if you can correct it or if you
just have to stop loading it.