How stepe by step to restore my dell dimension 8400 desktop computer without the cd.
Window xp professional

First, you don't without a Dell recovery CD or Windows XP CD.

Second, if you've never rebuilt a computer before, a detailed step-by-step process covering every single thing involved at the most basic level would include hundreds of steps.

Suggest you call Dell support or take computer to local repair facility.

This is called a "system recovery"
I do not have the disk either. My windows XP, was already loaded on my computer, when I bought it. These instructions, will clean everything off your hard drive, so you must back up anything that you want to save. I have done this three times, and I got these instructions, from a computer tech. When starting your computer, tap the F-10 key. Click on next-yes-and follow the instructions, that come up on your screen. When you are finished, your computer, will look just like it did, when you first bought it. Do these steps, in order, when finished.
1. set up windows firewall
2. windows update (click express)
3. install your anti-virus program (update)
4. install anti-spyware (update)
5. Install service pack 2... Good Luck..

F-10?

And I would install SP2 before doing anything else. And everything else is personal for you. Everyone has to install whatever software they choose to install, not a list given to you by some tech. And while a firewall, AV and spyware are a good idea, some people use 3rd party firewalls, and some Dells come with some of that already on them, etc.

And it's a terrifically good idea to manually download windows updates and install them manually yourself, since you can get viruses just by getting online. Getting online and getting the updates prior to installing SP2 sounds like a lousy idea to me.

chiquitalinda, I just went to Dell's site and looked in the support section for your computer. I entered your model and the XP Pro, so these instructions should work for that. It actually started at a page that is like a pre-check. I think it's a little lacking, since backing everything up could be very complicated. But it starts here:

Dell - Support (http://support.dell.com/support/topics/global.aspx/support/dsn/en/tree?c=us&cs=19&l=en&s=dhs&treeid=FFA33DA99941FEDFE030030ABD62041A&journalid=4CF38FF656D40D94E040A68F5A280503&Query=&SystemID=&ServiceTag=&contenttype=&os=&component=&lang=&doclang=en&toggle=&dl=false)

You'll have to follow all the pages through because when I tried to link directly to the restore page, it just came back to this one anyway.

I'll repeat again that I entered your model and XP Pro, so theoretically this should be it, but I can't guarantee that. I don't even know if this requires a disk. It doesn't mention it, which means the restore has to be somewhere on the hard drive, perhaps on a D drive. (Like Compaqs at least used to do.)

I got an XP disk with mine, and all the driver disks. So I do a normal install, not a factory restore. I hate those factory restores. I like a clean install where I can do everything the way I want, and NOT put it back the way it came from the factory. In fact, inside of two weeks of getting my computer, I wiped it out and installed everything from scratch the way I wanted it. :-)