combobox SelectedIndex set back to -1

I have an application written under VS 2010, OS Windows 7.
This application currently is executed on a XP box, until a new box
is installed, which OS is Windows 7.

A combobox is loaded with data on a form, and the user accesses this
data by dropping down the droplist and selecting an item,
or typing in the combobox text area until the correct
item appears.

Under Windows 7 the selection of the item either way works fine.

Under XP, the selection of the drop down box, works, but
typing in the combobox text area until the correct
item appears works until the user adds/update screen
data. I've setup displays while user types, and the
selectedindex is correct. But once any other button, function, event
or anything is performed the selectindex field for that combobox
is -1. Under Windows 7 it works fine.

I checked the .dll file version under 7 and xp, and both are the same product version. I am dumfounded as to why this is happening.