Desks that can raise and lower themselves automatically are awesome, but they're usually prohibitively expensive. If you aren't afraid of rolling up your sleeves though, you can make one yourself.
Instructables user Jwilcott walks you through his process. You'll need some linear actuators, a power supply, and some other bits and pieces, but you may be able to build the legs into your existing desktop, if you don't want to start completely from scratch.
It should be noted that the process is fairly labour-intensive, and those actuators aren't exactly cheap. Before you take the plunge, it makes sense to try out a cheaper DIY standing desk to see if you like it first. If you're ready to take your office to the next level though, this is certainly worth a try.
Electric Height Adjustable Desk [Instructables]