Is there any way to set my own index values for a particular list box?

Could you be a bit more specific as to what you mean? Also, what ultimate result are you trying to achieve - there may be another route to it?

Simon

My list box is populated by a query, and sometimes there can be more than 200 values! I get an error message saying that the value list is too long. Also, I need to be able to multi-select as some point.

Although it is not advisable to create listboxes or comboboxes based on A LOT of records, I have a few ones based on more that 1000 records working without any problem nor significant delays.
I do not see the relation between your index question and your too many records problem.
However, as to the number of records, check the limit set under one of the tabs in the menu Tools -> options and increase it if need be.

Alex