For the dates of purchase you can enter "Various" which means that you purchased the shares at different times. You may want to contact the institution that was handling the automatic reinvestment program to see if they can provide any information on your purchases. You could also request a record of all dividend amounts and the reinvestment prices that have been paid from 1990 up until 2007. You could then try to reconstruct a history based on the number of shares you had and the number of shares that would have to have been reinvested from your original shares. You do not have to submit proof of your cost basis with your tax return. However, if you are audited then you may have to provide documents that justify your basis. Good luck.
See pages D-6 and D-7 - http://www.irs.gov/pub/irs-pdf/i1040sd.pdf