Change the default tab order for controls within InfoPath

When filling out a form, many people use a keyboard, instead of a mouse, to move from field to field. Therefore, it's important that controls in your form have a logical tab order (tab order: The order in which the focus moves in a form from one field or object to the next as you press TAB or SHIFT+TAB.).

By default, the tab order in a Microsoft Office InfoPath 2003 form is left to right, top to bottom, just like it is on a Web page. If you need to change the default tab order for your form, use the following procedure. For example, you can specify a custom tab order for a repeating table (repeating table: A control on a form that contains other controls in a table format and that repeats as needed. Users can insert multiple rows when filling out the form.), so that users can navigate column by column rather than row by row.

1. 
   In design mode, right-click the control for which you want to change the tab order, and then click Control Properties on the shortcut menu.
2. 
   In the Control Properties dialog box, click the Advanced tab.
3. 
   In the Tab index box, specify something other than 0. For example, to specify that users navigate to this control the first time that they press the TAB key, enter 1 in the Tab index box.
4. 
   To change the tab order for additional controls, repeat steps 1 through 3.

Notes:
The default tab index setting for all controls in a form is 0, but the tab order starts with 1. That is, any control with 1 in the Tab index box will be visited first when users press the TAB key. Any control with 2 in the Tab index box will be visited second, and so on. Any controls with 0 in the Tab index box will come last in the tab order. If you want to skip controls in the tab order, enter -1 in the Tab index box.

Changing the tab index setting doesn't change the order in which you tab through controls in design mode. To test the tab order in your form, click Preview Form on the Standard toolbar, and then use the TAB key to move through the form.

from http://office.microsoft.com/en-us/infopath/HP010984681033.aspx