Abstract: Methods and apparatus for detecting a computer virus on a computer, by means of repeated execution of detection algorithms to detect presence of a virus. A multitude of detection algorithms are provided, and each time the computer is checked, at least one of the algorithms is selected and executed. Even if a virus could not be detected by a few of these detection algorithms, such a virus still has only very limited chances to survive because it will be detected by another algorithm. The invention gains additional security due to at least one detection algorithm which at some suitable instant compares the total of occupied and/or free random access memory with nominal values valid at that instant. Another embodiment includes at least one detection algorithm that provides special secure disk access functions to the operating system which are not publicly documented and which require additional parameters that must match their nominal values in order to enable the function.