What Causes CPU Usage to be at 100 All the Time?

By Keith Evans

A Virus

In a Microsoft Windows operating environment, central processor unit (CPU) utilization at or near 100 percent may be caused by a number of factors. The most common factor, however, is a computer virus. Operating in the background and not giving any overt indication of their residence on the machine, virus programs clandestinely eat up system resources as they generate thousands of spam email messages, monitor system activity, log keystrokes or perform any one of myriad malicious operations. Virus software is often sloppily written, and redundant code in the program itself will slowly (or instantly, in some cases) increase system resource utilization until the CPU usage maxes out at 100 percent. When this maximum is reached, the computer is so busy handling the operation of the virus that other programs slow to a crawl or stop altogether. To resolve issues with viruses, users of infected computers can download free software like AVG Anti-Virus, or purchase commercial packages like Norton Anti-Virus or McAfee Virus Scanner.


Similar to a virus, spyware is a covert computer program that quietly loads itself into memory and monitors the computer's usage. Spyware is somewhat different from virus programs, though, in that spyware is not always malicious in nature; it is sometimes developed and deployed by legitimate companies to perform tasks related to targeted advertising or computer security. Spyware can be harmful to both the computer and the user, though, as devious programmers take advantage of the technology to steal users' identities or perform other illicit activities; malicious spyware of this nature is usually poorly written and may cause CPU utilization to increase drastically until the machine becomes almost unusable. To detect and remove spyware, users with Internet access may be able to download and run a free program like Spybot Search and Destroy.

Too Many Open Programs

While virus and spyware programs are the most common culprit of high CPU utilization, users with older computers or processor-intensive software may simply max out the machine's resources by overloading the processor. When newer operating systems--which may have numerous patches and enormous kernels--combine with data-intensive software like Computer Aided Design (CAD) or video editing programs, the computer's processor usage may spike to 100 percent as the machine struggles to keep up with the software and operating system demands. To resolve this issue may require upgrading to a more powerful computer, though a full virus and spyware scan should be conducted first to rule out any malicious software problems.