It probably happens more than you want: as a desktop or field support analyst, you're visiting a customer location to resolve a problem, having been dispatched there by the help desk. This visit is only one of numerous other tickets you have to resolve, and the clock is ticking. As you're working on that problem, a coworker of that customer taps you on the shoulder and says, "Oh, while you're here, could just 'take a second' and...?" (They always say it's "just a second," don't they?)
What do you do? Sure, you could simply go help that person. But of course, a second becomes a minute, which becomes a half hour and so on. And what's to keep someone else from tapping on the shoulder while you're with that second customer? In the meantime, what happens to that list of assigned tickets and customers you were supposed to be visiting? How happy will your boss be about your being waylaid this way? On the other hand, what if you were to blow that person off? You'd be closer to being on schedule with your tasks. However, you run the risk of alienating the person who asked for your help, causing that person to complain to your boss.
This situation reminds us of the mythical monsters Scylla and Charybdis, who sat on opposite sides of a narrow strait of water. If a ship that passed through this strait came too close to either side, it risked destruction. The phrase "between a rock and a hard place" probably arose from this myth.
In this type of situation, where you'd like to give full support to that customer but can't, examine all your alternatives. Maybe you could suggest a solution that helps that customer at least somewhat, while at the same time keeps you close to schedule. What if you set a time limit, for example "I can spend five minutes looking at the problem, but if it takes more time, I'll have to ask that you call the help desk." If time really is an issue, could you offer to make the help desk call for them, or else stay with them while THEY make the call? In other words, can you think of any action, no matter how small, that would make that interrupter better off?
If none of these alternatives is appropriate, and you can't think of any others, avoid simply telling the customer "No, I can't help you." Even though the statement is true, saying it this way risks alienating the customer. It's usually better to show some empathy, in the form of regret. Therefore, consider saying, "I'm sorry, I'd really like to help, but I have all these other tickets I have to handle." Then, continue with "If I were you, I'd call the help desk so that they can log the problem and get someone out here to help you." This approach still may cause problems, but the chances are less than with the simple "No, I can't help you."
How do you yourselves handle the drive-by interruption? What has worked, and what has created problems?