There are a couple ways to approach this depending on what you are going to end up doing.
In you turn on smart navigation for the page, ASP.NET will automatically scroll the page and focus onto the control that was last active. However, based on what you said about the textboxes this might not work perfectly. Assuming that your postback is being caused by a textbox change even, you have to leave the textbox for that clientside event to fire, which then fires the postback. So your return focus might not be exactly where you want it.
: document.getElementById("<value from clientid>").focus();