On a good day, high CPU (central processing unit) usage will slow down your computer system, still allowing you to complete your tasks, though not allowing you to be efficient about it. On a bad day, high CPU usage can halt your system entirely, forcing you to manually shut down and restart your computer. There are several causes of high CPU usage, all of which are avoidable and, with the proper tools, even preventable.
Malware comes in many different forms---virus, Trojan, spyware, worm---and can increase, or even max out, CPU usage. Removing malware can be tricky business, though is best done when first noticed and before it has the opportunity to create worse problems. Once malware is removed, CPU usage will be greatly decreased, if not returned completely to normal. The best way to stop malware, though, is to install and regularly run a trusted antivirus/antispyware program.
Many computer users multitask without even realizing it. They're playing songs on iTunes while surfing media-heavy pages on the Internet, writing a paper, running antivirus software in the background and allowing several programs (such as Adobe or Windows) to listen for product updates. It's simple common sense: The more programs a computer uses, the higher its CPU usage will go. And if you're the kind of user who must be multitasking, tweak your system configuration (msconfig.exe) to halt any unnecessary applications that run at startup (see the link in Resources for how to do this).
Malfunctioning or Incompatible Software
Some programs are too advanced for the likes of Windows XP. Some programs predate Windows 7 and have no compatibility patch. Other programs have certain hardware requirements (graphics card, RAM, processor speed). And occasionally, a program can simply become corrupt. In some cases, these programs will not open or run. But in other cases, these programs will utilize any remaining CPU resources in an effort to function. The solution is to make sure your software is compatible with your system's operating system and hardware specs and to remove that software if it is not.