Windows License.

I was recently browsing through old software at the office my mom owns and found a windows XP pro upgrade disk, and i thought it would be pretty cool to have because i can get pro stuff like group policy but keep my programs. Upon reviewing the license agreement it says ONLY USE THIS ON ONE WORKSTATION blah blah blah. and it was legitimately purchased and everything but i think its been used before. and if thats the case what will happen if i install it? because it says i have 30 days to verify it online OR ELSE. and i honestly dont know what or else means because i havent tried it before. i can only assume it goes nuclear and destroys 1/3 of the world. Anyone else try it before?