Serafin, it is worthwhile to persist with your attempts at reflashing the software. According to Ima (one of the most knowledgeable members of the forum), you will be able to boot your N800 after reflashing, although the lock code will not be reset. (See http://talk.maemo.org/showpost.php?p=589207&postcount=4) You will have to be careful not to lock the device again accidentally before you find the lock code. Instructions for finding it are later in this post.

You do not have to use DOS commands if you follow Reinob's advice and use the Nokia Internet Tablet Software Update Wizard. Click on the second link in his post, and then scroll to the bottom of the europe.nokia.com page, where you will find a link for downloading the Wizard. (DeBernardis gives a link to the Italian version of the same page. Il link rilevante "Scarica" si trova al fondo della detta pagina.)

Any of them will work. If you want to use your N800 without many further modifications, your best choice is the last but one, i.e. RX-34_DIABLO_4.2008.36-5_PR_COMBINED_MR0_ARM.bin .
You can always change to the last version at a later time, if you decide that it will be better for you.

HOW TO FIND THE LOCK CODE

Once your N800 has booted and you have connected to the internet, install an application called "rootsh". (If you need help to do this, just ask.)

There is an application called "X Terminal" among the built-in "Utilities". Open it and type the following line:
(You can find the "|" symbol among the upper-case items of the onscreen keyboard.)

Press the "line return" symbol (sometimes called the "enter" symbol).

The lock code will appear on the next line. Most probably it was set by a previous owner, before you bought the N800 on eBay. I have had that difficulty myself.