I have written about how sitting is horrible for you before. Basically, the more hours of the day you spend sitting, the shorter your life expectancy. This means that most of us are shortening our lives, just by doing our jobs. The alternative is to stand while you work at a computer. this is something that be easily done. Just elevate the computer high enough to make it at a level that is comfortable to use while standing. You don't want to be looking down at it; that will strain your neck. You want the screen to be directly in front of your eye-line and your keyboard at a level that is comfortable.
The hard part is going to be doing this at work. I'm sure that you can come up with a way to get all of the elements to a comfortable level. Using copy paper boxes and other items, that can be done. What will be hard is explaining to your co-workers and boss why you're doing it. I sat on an exercise ball for years, and had to constantly defend that choice to the same co-workers repeatedly.
I think that showing them the latest study on the harmful effects of sitting may help. However, I can't find any OSHA rules about desk set-up. They show how a desk should be set up for the best ergonomics, but I don't think they require that businesses to actually follow those recommendations. It may be difficult, but seems to be worth the struggle.