Jan used to work in Corporate HQ in New York, until she decided to go crazy, get a boob job, spend half her day online shopping,
and constantly visit her sister in Scottsdale (aka, visit Michael). Jan initially had nothing but disdain for Michael, as he is an excellent
salesman, but extremely inept manager. 
She once had Pam take a note of everything that Michael did in his regular work day to see if
Dunder Mifflin could justify his salary.
Jan and Michael scored the Lackawanna County paper account together, mainly due to Michael's impressive sales and people skills. She kissed him, and they retreated to a hotel after that. Although Jan said she didn't want anything more to do with Michael, Michael did not give up pursuing her, and it took some rude words about how awful Michael was before he gave up (but not before the whole office found out).
Jan and Michael then went to Jamaica after Michael was dumped by his girlfriend for Photoshopping an image of himself over her ex-husband in a picture she had.
Michael accidentally emailed a picture of 
Jan topless, with Michael behind her, which led to an uproar at Dunder Mifflin (especially when the picture was
enlarged several hundred times and displayed at the warehourse).
Jan was pretty abusive to Michael, so they did break up, but the minute she had her breasts enlarged, he got back together with her. Naturally, once they got back together, Jan became abusive again, especially after she became a live-at home candlemaker and destroyed Michael's 10" plasma screen TV. Jan moved out, but recently artifically inseminated herself and had a baby, much to Michael's horror, who felt that she cheated on her, especially after he "specifically asked you NOT TO!"