Computer Senility (or "droid rot") is the umbrella term given to a range of conditions that considers how a sentient computer might develop mental instability if left alone for millions of years. Computer senility would therefore be a product of lasting loneliness. However, an attributing factor is likely the deterioration of the Holly Room.
The sentient computer in question is Holly. Initially having an intelligence quotient (IQ) of 6,000, Holly becomes very strange after Red Dwarf has drifted through Deep Space for three million years. An early demonstration is his tendency to completely speak his mind, as when he points out the age difference between Dave Lister and the pile of dust that was Kochanski, admitting that he no longer had any social cues and that he'd gone a "bit peculiar". (RD: The End) One of Holly's odd activities alone was the invention of Hol Rock, a form of decimalized music with two extra notes that Holly invented, "H" and "J".(RD: Kryten) He also claimed to own a collection of singing potatoes to keep himself sane, although this may have been part of his sense of humour. (RD: Queeg) In the novels, Holly was alarmed that he was losing a lot of trivial information due to his senility. One of the results of his senility was that he was starting to have unshakable faith in Silicon Heaven, a concept he dismissed when he had his original IQ but developed when his IQ reached the low nineties. Other symptoms that Holly showed was a blindspot for the number 7 and losing a lot of information he should know.
Holly would came up with extremely bizarre ideas which would at first give hope, such as serving a two year prison sentence in just fourteen weeks. Unfortunately for Lister, he'd have to become a dog. (RD: Cassandra) His current IQ had been reduced to only 68. Holly brings Lister out of stasis partly because he is so lonely, and partly because the radiation from Red Dwarf's engine failure had dropped to a safe background level. He himself considers that he may have gone 'a bit peculiar' left on his own for so long. (RD: The End) His behaviour became so erratic that he changed his face to that of Hilly, his female opposite he fell in love with in a parallel universe.
Holly briefly regained more than twice her original IQ in a scheme devised by Kryten and tested on Talkie Toaster, essentially compressing her remaining intellectual abilities into a shortened lifespan, but this left her with barely two minutes of run-time until a local temporal anomaly erased the experiment completely. Holly wasn't the only AI to suffer computer senility. Kryten claimed that he knew a fellow Mechanoid who was senile, whose name was Gilbert but preferred being called "Rameses Niblick the Third Kerplunk Kerplunk Whoops Where's My Thribble". (RD: White Hole) Hogey the Roguey was a Simulant the Dwarfers found non-threatening and ridiculous; Hogey was seemingly also suffering from a form of droid rot judging from his bizarre speech patterns, erratic behavior and obsession with duels across time and space. (RD: The Beginning) Kryten's brother Able had serious droid rot, but this was more due to drug abuse than the passage of time. {RD: Beyond a Joke)
Other artificial intelligences have also seemingly shown forms of computer senility, such as Gordon. (RD: Better Than Life) Kryten's replacement Hudzen-10 had also "worn down his sanity chip" after spending almost three million years alone looking for Kryten in the Diva-Droid Shuttle, resulting in him being willing to attack anything standing between him and his goal of destroying Kryten ('justifying' his attacks on the grounds that the Cat and Rimmer weren't human and Lister barely qualified). (RD: The Last Day)
Kryten's Spare Head 3 suffered from a condition specifically known as "droid rot". This seemed to manifest itself in outraged rants against Kryten, and behave like a surly Yorkshire man, even using idioms and slang from that area. This condition was the result of 30,000 years of his circut board going "bandy" and voice iunits "shot t'buggery" (RD: D.N.A.) A vending machine on Red Dwarf also had a defect in its voice recogition unit. It spoke with a lisp and got black coffee mixed up with a bucket. (RD: Future Echoes)