What Parts do i need to buy?(whats missing)

How can we possibly know what's missing? You may need to post pictures inside the case so we might be able to see what's in there...

You can buy any hard drive you want from anywhere you want. You don't have to buy it from Dell. Price will depend on the size of the new hard drive (GB).

And edit your post to remove your Service Tag. Never post the Service Tag in a public forum.

EDIT: @JOcean did it to me again...! 
EDIT2: Any photos you post won't be visible until the moderators review them, even if you can see them. So we may not see them before Monday...

Do you have a picture or description of whats missing? If you go to Dell support and type in the service tag number you can look at configuration and see what components were originally installed. As far as an HDD is concerned, they are all very inexpensive now for a 500 GB or 1 TB drive. Keep in mind that unit is approaching 13 years old. So the newest drivers supplied by Dell are for Windows Vista. If you install anything newer Windows may auto install some or all of the drivers or you may have to search the Internet for those drivers you require.

EDIT: Sorry..we were taking down Christmas and reorganizing the house...I had energy to burn..

@JOcean   Don't be sorry. At least we said the same thing... I got an "authentication" error the first time I posted, so by time I got that fixed your post was there...

I do have pics & was gonna post but didn't know how. As u can see idk ANYTHING about computers or forums for that matter. Thank u for the tip about the tag.. again no clue what I'm doing.. I did type in & I have not a clue what's missing or there all I know is the dude told me it needs a hard drive & windows 10. I paid $20 for it, figured wth if I can get it running for my 10 yr old then awesome if not then I didn't lose anything unless I buy stuff & it don't work. I'll post pics & I appreciate any feedback. All he wants to do is play games on it..I can't afford to buy him a gaming pc so I'm hoping I can surprise him with something. Thanks again.

To post a picture in your post simply click on the camera icon above the response window, then browse to the pictures on your pc.

A moderator has to approve them first. But let me take a stab at something. A list of what any desktop needs.

This web page is very informative with pictures and a list of required parts. Possibly with this information you will have a better idea of what you need and may be missing.

Plug it in and turn it on. See if you get the bios screen complaining of no boot device . If you do, pick up a cheap SSD plug it in and use some velcro to stick it to the bottom of the case. Install Windows.

XPS 420 runs Windows 10 fine using Microsoft's generic drivers. Still this is an old computer that might be stripped of other crucial parts. I agree that it's not worth spending much or any money on it. It was discarded for a reason.