It's definitely possible to lose weight in a week, the only catch is how much. Much of it depends on your body type, how much you weigh to begin with etc, but it should theoretically be possible for an overweight person to lose 5 or 6 lbs in a single week. Bear in mind that sex will come into play though, as men can lose weight faster than women, due to their increased muscle mass which means they can burn calories a little faster. It should still be possible for just about anyone to lose weight in a week though.
Set yourself a realistic goal. If you want to lose 22 lbs in 7 days to get into a dress for a party or whatever, you don't have much chance. I'm sure it could be done, but I'm not sure if you would still be living after! Monitor your weight over the whole time, and set yourself checkpoints along the way - If you want to lose 6lbs for example, you know you have to lose just under a lb a day. Also, doing this will stop any nasty surprises and help you adjust things on the way to hitting your big goal, and gives you more chance to lose weight in a week.
Find an exercise you like and get used to doing it. If you want to lose weight in a week, seriously lose weight, exercise is essential. Now I understand that there may not be an exercise you like, but try and think along the lines of something that you find the lesser evil. In order to lose any real weight inside a week, you'll need to be doing this exercise pretty much every day! Something that's high in activity is going to be best, jogging or cycling, and if you can only bring yourself to walk, you had better plan some long hikes!
Don't keep it a secret. If you admit to a friend or family member that you are trying to lose weight in a week, it's likely they won't believe you are going to do it. What better motivation for proving them wrong? Also do you want to have to face up to them and tell them that you failed? Things like that can be great motivators, and bear in mind, motivation is exactly what you need to get you really working out hard and watching your diet!