Before he became the top U.S. tax collector and the target of an unprecedented Republican-led impeachment drive, John A. Koskinen majored in physics, guided the country through the "Y2K" problem and ran a foundation that promotes youth soccer. Now, amid a three-year controversy over the Internal Revenue Service's scrutiny of conservative groups, Koskinen, 76, is being re-educated in the exertion of forces, the limits of computer technology and -- according to a predecessor -- the sensation of being kicked around.