Is MS Office XP Pro available to download online

Well there is Office XP Professional from 2002 and there is Office 2003 Professional from 2003.

But regardless of which you have, the licensing terms for both are the same with respect to the question you asked.  Retail licensed Office XP Pro or Office 2003 Pro can be concurrently activated on two of your computers, usually one being your main computer and the other being your portable (laptop) computer.

Since you already have it concurrently activated on your desktop and one laptop, you have maxxed out your allowed activations.  The retail license does permit you to move one or both of these activations to different computers, you just can't have more than two activated at any given moment in time.  So you could activate it on the netbook, BUT only after uninstalling it from either the desktop or the laptop.

The Microsoft Office "student" product permits three concurrent activations within the same household.  AFAIK there wasn't an Office XP student product.  For Office 2003 the student product was called Office 2003 Student and Teacher edition, and for both Office 2007 and 2010 it's called Office Home and Student yyyy.

There is no legal download of either OfficeXP or Office 2003.

You activate in the usual way, via internet.

If you have a netbook, with no cd/dvd drive, how do you plan on recovering that system when it all goes wrong?

You copy the complete installation disk to thumb drive, or network the systems, or buy an external cd/dvd drive in order to install