A 30 year old man since 2 months suspects that his wife is having an affair with his boss. He thinks his friend is also involved from abroad and is providing technical support. He thinks people talk ill about him. His friends tried to convince him but he is not convinced at all. Otherwise, he is normal, he doesn't have any thought disorder or any other inappropriate behavior. The most likely diagnosis is -
Persistent delusional disorder is characterized by non bizarre delusions (i.e. involving situations that occur in real life e.g. deceived by spouse) of at least 1 month duration (DSM IV) but with absence of :

Prominent hallucinations & flattening of affect
Thought disorder, mood disorder, inappropriate behavior
Cognition, memory & consciousness impairment
Paranoid personality disorder
Persistent delusion disorder
Schizophrenia
Acute and transient psychosis