He was granted the County of Cotentin—the peninsula that extends into the English Channel and contains Cherbourg and Bayeaux—for £3000 from Robert, slowly establishing power and some authority. Robert had hoped to be given England as well as Normandy, and wanted Henry on his side. Since Henry was otherwise landless—which at the time meant having no power whatsoever—he allied himself with Robert. Robert's intention to take England from William never turned to action, however.
In 1088, Bishop Odo of Bayeaux convinced Robert that Henry could not be trusted. Odo seized Henry, who was captive for the winter; Robert took back the Cotentin. In spring of 1089, nobles in Normandy persuaded Robert that Henry should be released.
In 1091, William invaded Normandy, defeating Robert and signing a treaty with him making each the other's heir and completely leaving Henry out of the negotiations. Henry decided to fight his brothers, but wound up being besieged in MontSaint-Michel for a time. Rumor has it that, when Henry ran out of fresh water, Robert allowed supplies to be taken in, upsetting William Rufus. The back-and-forth between siblings in that family would surprise no one who had seen a certain movie based on their descendants, The Lion in Winter.
Time passed, and once again William and Henry were on amiable terms. When William Rufus died on 2 August 1100, Henry "happened" to be present (click the link to understand the quotation marks). Henry wasted no time in getting himself crowned. Although rightly the throne should have gone to Robert, England accepted Henry with alacrity because of the Charter of Liberties, in which Henry made promises that undid some of William's unpopular practices. The nobility might have been fed up with the constant fighting between Robert and William and embraced Henry as a sort of "compromise candidate." After some debate over the rightful heir, Henry was crowned 5 August in Westminster Abbey.
Three months later, Henry married Matilda, the daughter of Malcolm III of Scotland, also known as Malcolm Canmore, in a shrewd political move. That Matilda might have been a nun did not deter the marriage. They had two children: Matilda and William Adelin, who died in the White Ship tragedy.
Marriage did not mean fidelity: kings were understood to exert their sexual prowess and desires in many directions. Henry had at least ten acknowledged extra-marital children who lived long enough to have titles and careers. Matilda of Scotland died in 1118, and when the White Ship sank in 1120 taking his legitimate son with it, Henry collapsed with grief. Now with no legitimate son, and seeing the prospect of numerous less-legitimate heirs and various nephews who could tear the country apart fighting for the throne, he decided to marry again.
So finally we come to the first royal wedding to take place in Windsor Castle, to Adeliza of Louvain. See you tomorrow.