A computer can freeze, or become unresponsive, due to age, memory issues, hard drive problems, hardware conflicts or processor problems. It can also freeze when it is infected with viruses or other malicious programs.
If a computer has been running for many days without a reboot, virtual memory can become clogged and slow down the machine. When multiple applications are running on these machines, there comes a point of time when conflicts occur and the machine freezes. Other common reasons for an unresponsive computer are registry issues, badly fragmented hard drives, conflicting remnants of old programs, a backlog of temporary files and security software that steals system resources.