I have a temperature/humidity sensor that connects via USB to a PC. There is a program that reads it and displays the data. Is there an easy way to get the Pi to read this data and get it into an Open Office spreadsheet? Can it be programmed to do this on boot?
Drivers are the big thing. If you can find or write Linux drivers for your sensor, you should be able to make that happen. Once it is recognized by the Raspberry, your only limitation is the API (Application Program Interface) that the manufacture of the sensor provides (or that you write into your driver).
You can then take that data that is produced, and with a small, simple script, dump it into any form you want. I would recommend a CSV file as well, as it is simple to create, and simple to read into any program you want.
Apologies for this answer being so broad and all encompassing, but you didn't give a whole lot of details to work with.