What is a common maintenance step when a computer is running slowly?

Defragmenting the hard drive is a common maintenance step for addressing slow computer performance. Over time, files on a hard drive can become fragmented, meaning they are stored in non-contiguous blocks. This fragmentation can cause the computer to expend additional time and resources to read files, as the read/write head must move to multiple locations to access a single file. By defragmenting the hard drive, the system reorganizes the data, arranging files in contiguous segments, which allows for quicker access and generally improves system performance.

The other options focus on different aspects of computer maintenance or upgrades. For example, restoring the operating system could help in specific situations where software corruption is causing issues, but it is not a routine step for slow performance. Upgrading the monitor would not significantly impact the computer's speed; it primarily enhances visual output. Cleaning the keyboard can improve the physical usability of the computer but does not address performance issues. Thus, defragmenting the hard drive stands out as a direct maintenance step that enhances system efficiency and speed.