Disclaimer: Parts of the following have been inspired by a re-reading of Cyril Hart's Between Severn (Saefren) and Wye (Waege) in the Year 1000.
Aethelred II was born about 968AD, son of King Edgar the Peaceful and his second wife Aelfthryth (otherwise Elfrida). His name meant "Well-Counselled". He was crowned in 979, at about the age of 10, after his elder brother Edward the Martyr had an accident at Corfe Castle (he accepted a drink of mead from his step-mother, and one of her men stabbed him, in the back).
In about 985 Aethelred married Aelfgifu of Mercia (who may have been the energetic young woman who had previously bed-shared with Edwy All-fair and her mother, at his (Edwy's) coronation) and they had eight children, one of whom, Edmund, was later to be king (Edmund Ironside).
Unfortunately Athelred was also plagued by Viking invasions, who systematically plundered the Wessex coast from 997AD on. The King could either "pay or play" -- he could either bribe them to go away or he could fight them. Regrettably he lacked for both ready cash and an efficient army. He also lacked for counsellors with, frankly, spine, to advocate one course or the other through thick and thin. Thus he gained his later byname of Unraed -- the Uncounselled, or the Ill-Counselled (perhaps also became of his selection of Eadric Streona as a son-in-law and chief counsellor -- a man later to be demonised as a multiple traitor).
The Millenium seems to have spurred him into action -- he found an army and a fleet, and went to war against Malcolm, his Cumbrland under-king, who had been remiss in contributing to the dane-geld taxes. Two years later he sorted out a dispute with the Duke of Normandy by marrying the duke's sister Emma, the Pearl of Normandy. Their first son, born within the year, was to be Edward the Confessor.
It was the same year that Aethlred decided to solve the Viking problem by having all Danes living in England killed.
In 1006 the Vikings returned; the next year Aethelred bought them off; in 1009 they returned. In 1012 they martyred the Archbishop of Canterbury at Greenwich, outside London. In 1013 they invaded in force and Aethelred fled for his life to his brother-in-law, Robert of Normandy, leaving England to Sweyn Haraldsson ('Forkbeard'). He died in 1014 and Aethelred returned, to reclaim the throne, and died in 1016, being succeeded by his son Edmund.
His 37-year reign was, in fact, one of the longest in English history, and introduced several long-standing features: the organisation of shires and sheriffs, and the introduction of the 12-man jury, for two.