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Unless you download a new beta from MS you cannot see RAW files in Windows, and in Adobe PS CS, you must download the current version of camera raw from the Adobe website. As new cameras come out, Adobe adds them. The 20D was added last Fall. You also got some software with the 20D from Canon which will read them and convert them to TIFF or JPEGS.
If you are not familar with RAW processing, get a copy of Bruce Fraser's Camera RAW and Adobe PS CS or CS2 and read it carefully. It will give you a lot of information you need and is excellent.