I have a rather large spreadsheet (huge model) that reads and writes
from dozens of other files - and shortly after it opens, it will
disappear from the taskbar at the bottom of the screen. Excel is still
open, but just not accessible from the toolbar - you have to press
Alt-Tab and pick the right Excel icon to get to it.

Most users are running Windows XP (some on 2000), and all are using
Office XP with SP 2 or 3.

I am thinking I have done one of a few dangerous things (I can't paste
the program logic, because it is over 9,000 lines, and I don't know
where it is happening):

Closing these miscellaneous files after I have read the data with
ActiveWorkbook.Close

I open 3 files - one is used, the other two have values read by the
main file, so I hide them with statements like (parts file is a
variable with the name of a spreadsheet that stays open, program is the
name of the program that is running):
Windows(PartsFile).Activate
Windows(PartsFile).Visible = False
Windows(Program).Activate

I added (if it would help)
Windows(Program).Visible = True
Sheets("Country").Visible = False

periodically, I will also turn on and off screen updating
(Application.ScreenUpdating = True/False)

Does anyone have any idea why Excel disappears? I have some new users
coming on board who are nervous about the screen disappearing, and
since they run the model 50 times a day, this is a major annoyance.

Thanks,
Brandon