As Kees says, the machine is not worth a great deal commercially. To put that in context, I bought a Lenovo T500 Thinkpad laptop last year, four years old, Core 2 Duo 2.53 GHz, 15.4" screen in pristine condition for the equivalent of US $30 - yes, thirty!. Didn't have an operating system but that's OK, I wanted it for Linux.
So what about yours? While its commercial value is very little, it may have some value to a number or places. Maybe some youngster wants to learn PC basics hardware - what better place to start than a desktop that isn't going to break the bank if (s)he breaks it. Or maybe a school with an electronics lab, if they have them where you live. Or there are the charities that recycle old technology for the third word - some of the older components, like the hard disk and the memory are extremely difficulat to source - it could make the difference between a recycled machine and the scrap heap. So unless you've done so already, I'd say don't destroy the hard disk.
But what you MUST do is clear it. DBAN has already been mentioned and I use Partition Wizard from Minitool (free for the home edition and the bootable CD). WHat you want is the bootable CD. Make absolutely sure that you have copied everything you want off the hard disk, because once cleared, it's gone and no way back!
Download the bootable CD image (.iso) and burn it to a blank CD - note burn it, not copy it - you want to recreate the system in the iso image. Then boot up the CD and it will display all the partitions on your hard drive. From the tone of your question, I suspect there is only one but if there are more follow the same routine for each. Select the partition and either on the left hand menu or partition drop down menu, select Delete. It will ask you if you are sure (yes) and if you want to do a secure erase (yes). It will then delete the partition and overwrite the space it occupied with ones and zeros. Your old data is now gone forever. You can either stop there and dispose of the machine with the wiped disk or you can create a new partition - accept all the defaults and it will be one partition across the whole disk.
If you feel you want to have a working machine to dispose of, you'll be doing nobody a favpour reloading Windows XP, so betore you wipe the disk, download one of the Linux distros, Mint or Ubuntu might be good choices and burn that to a DVD. After you've wiped the disk, boot the DVD and answer the usual questions and it will install a clean Linux system to play with. You might even like it! But at least all your data is cleaned off.
Check the charities to see if there is one in your area, the schools or the free community sales sites like Gumtree and see if there is any interest in the machine. Locally is best, you don't want shipping costs!
Last resort, your local council hazardous waste disposal depot - should be free in most locations.
Thanks for your question, hopefully the replies will assist your responsible disposal.