The first step is : you need to have installed a virtual box.
In Ubuntu 14 onwards, it is available in the software centre so it is better that you install your virtual machine from there and open up the virtual box and click add new.
It will ask for ram size and allocate it according to your computer ram. Remember, 512 MB is minimum for Windows XP and 1024 MB is better.
Next it will ask for the file location and storage hard disk. Allocate this accordingly. Minimum 30 GB.
Now the virtual disk will be created.
Next either you put an ISO file or boot-able CD or DVD in your CD/DVD ROM. and link with after clicking the “Start” button above.