Measure the axle-to-crown length. This is from the center of the dropout, to the bottom of the lower headset bearing. Your replacement fork just needs to be close, it doesn't have to be exact. You will need to look for one that is 'suspension corrected' meaning it's got the extra length figured in.

The Nashbar Carbon MTB fork has a 385mm Axle-to-crown, and their Cro-mo 26" has a 453mm Axle-to crown. That's a difference of roughly 3 inches, which will affect things like the head angle, BB clearance, and basically rotating your whole riding position forward and down.
The description on the carbon fork states that it's intended for bikes that were originally full rigid, or had short-travel (<80mm) forks.

It also helps to know what you're dealing with other than a '10-year-old Trek' with 26" wheels. Model, pics, or description would help.