I don't know, but I would certainly try to get my data into an up-to-date
format, just in case, e.g. perhaps
a. create a new spreadsheet
b. cop/paste from the old spreadsheet into the new one.

You could also consider copying/pasting your data into a completely
different format for the purposes of the merge, e.g. copy/paste into a new
Word document.

800 records shouldn't be a problem .See if
http://www.gmayor.com/mail_merge_labels_with_word_xp.htm helps

Hi Lisa -
I had something like this happen and I had my "next
record" commands in the wrong places.  I was a bit rusty
and was putting "next record" before each word merge
command for example - in each label cell I had:
Next record <name>
Next record <address>
Next record <city,st,zip>
Instead of:
Next record <name>
<address>
<city,st,zip>
Merge was a mess -- I forgot that word looks from row to
row.  Just a thought that you might be doing the same
thing!-- Of course your first label cell you would not
have "next record".
take care & good luck!
Sue