I recently helped a friend to install XP on his vista computer. Here is what I did.
1. XP with SP2 (CD), If not SP2, you will have trouble to install XP on some SATA2 computers. You need to download SATA driver.
2. Goto http://www.csd.toshiba.com/cgi-bin/tais/support/jsp/home.jsp?nav=Download
Search the drivers for his laptop and download all the XP drivers to a USB (Most important is EthernetCard driver)
3. Format C:
4. Insert XP CD and install it (I prefer to have 3 or 4 partitions)
5. Install XP drivers.
It is very easy with Toshiba. But it is HP, then you need everest (http://www.lavalys.com/products.php?ps=UE&lang=en&page=9) to know what's in your computer and goto the product website to download XP drivers.