did update cause code to stop working

[carsto]

Code which has been running fine for months is now erring "out of range"

Running Excel 2000 9.0.8950 SP-3
recent updates:
- XP Hotfix KB884038
- Windows Imaging Component
- Hotfix for Windows XP KB915800
I have defragged.

The code works on other computers running Excel 2000 9.0.2720
It's not the workbook, the code doesn't work in any workbook.
These are not consecutive lines of code, just examples of error locations:

Error is 9 "out of range"

    If Workbooks("Book2").ReadOnly = False Then

    For Each ws In Workbooks("Book2").Worksheets

(ws = Nothing)

I can change Workbooks("Book2") to ThisWorkbook
but I'd like to figure out why it quit working

sample attached.

[carsto]

and it seems to be a potential nightmare.

All my problem code worked when I added .xls to the workbook name

In Windows Explorer, I had changed my Folder Options. I had UNchecked "hide extensions for Known file types".
The code started working again when I CHECKED it again.

How does one know whether or not a user will have this selected or not?
I don't see that most code is written with the .xls
How are others dealing with this?