How are you gauging the 'slowness'? In comparison to . . .?
Unfortunately - you seem to be running a fairly OLD system (if it has Windows XP on it) and without more details (make & model of the system, amount of system memory, etc.) it will be a shot in the dark to troubleshoot your issue, if it is not just a perception based off other systems that are newer/faster/supported.
Your system not using much CPU just means you are doing nothing that needs to use much CPU power. You cannot *make it faster* by using more CPU power - because if you aren't doing anything to use it in the first place - adding things to use the CPU would make little to no difference (might even make things slower.) The CPU power is there *if* you stat doing something that needs it (obviously) - but it is not your slowness issue nor can it be used to remedy it.
Given the age of the system (based off it having Windows XP), it likely has a slower hard drive, slower memory, less video card power and such than most modern systems. When compared to them running similar software - it *will* be slow.
Also - I have found in the years of doing this - most people's perception of their computer speed is not a true measure of the actual speed. Over time applications and such that the user wants to use have been upgraded and - for the most part - gotten larger and consume more memory and hard drive space and may even require more processor power to run as the upgrades occur. This contributes greatly to the feeling of a sluggish computer.
So . . .
What is the make & model of the computer in question?
How much system memory does it have?