"The Xbox edition of Serious Sam had notable differences including the model of Sam being changed from the model of The First Encounter to form a more cartoon style Sam, removing the shades to make him look less like Duke Nukem and more like he does today. The Xbox version includes all of the levels from both of the PC games. As a result, weapons from The Second Encounter were added to the levels that were part of The First Encounter on PC. The levels were made smaller by removing the areas outside of the playing area. Unlike the PC version, Sam can't exit too far out of the playing area. This is due to the smaller amount of memory available in the Xbox. The gameplay was given a more arcade touch. A life system was implemented into the single-player game making use of the score. Every 100,000 points grants the player a new life, which allows respawning at the place of death rather than at a save point. Combo points were added for killing multiple enemies at the same time, which allows the player to increase his/her score at a fast pace. Further changes to the game include the addition of treasure and the tweaking of save points. Save points are designated by a red phone booth. Auto-aiming was also added to the Xbox version to compensate for the degree of control usually offered by a keyboard and mouse setup."
Source: Wikipedia, "[en.wikipedia.org/wiki/Serious_Sam Serious Sam]", available under the CC-BY-SA License.