ERROR

Hi
I am using a spreadsheet in Excel 2002 on Windows XP, for some reason the
account numbers (which someone input, along with addresses) have a little
green mark in the corner of the cell. I thought it was to show that there is
a comment, but it is not. When you click on the cell, a little warning sign
shows, saying ERROR - number in this cell formatted as text or preceded by an
apostrophe. When I click on the arrow by the warning sign, it says
Number stored as text
Convert to number
Ignore error
Edit in formula bar
Error checking options
Show formula in audinting toolbar

Now I looked at the cell, and it has no apostrophe, so I went up to Format,
Cells, Number but that had absolutely no effect when I said to convert the
cells to number format. So I click on Convert to number and what I got was
something like this - 1234567895+E2. What is going on, I was told to just
ignore it that XP does weird things to Excel. But I want to know why this
happened in the first place.
None of the other account numbers I am putting on the SS in another column
are doing this, only the ones that were input by another person( I think this
SS was originally done in Access, but why the weird numbers)