It is surely possible as a Mac is just an Intel PC with different boot up system. We can install in a "windows" computer a boot loader which perfectly simulates the Apple EFI structure. My Intel laptop (2gb RAM, Intel Core 2.2GHz is dual booting Windows and Snow Leopard and has never given any problems. It's fast too.
I had the most success initially with iDeneb 10.5.6, then upgraded to 10.6 using a USB stick with the new Chameleon bootloader. You can use the Leopard installation to set up the DMG file and bootloader on your USB disk or memory stick. The small online upgrade to 10.6.1 is hassle-free. Do your partitioning and disk preparation from the Disk Utility in Leopard. Its easy.
Also check your BIOS for the USB and SATA settings, etc. Don't forget to make the partition format compatible and set correct permissions (I am using MBR rather than GUID). Note the install is slightly different (to install to an MBR you need a modified osxinstall.mpkg install file). When customizing during the initial installation only install the mimimum kexts, you can add wireless, sound etc later. I used the VoodooHDAudio and it's perfect. You can use OSXTools to get the video card EFI string and add it to your com.apple.Boot.plist if you want. Keep notes and update them as you go in case you want to change something or reinstall later.
The wireless card can be a problem if not supported (mine worked out of the box as did Bluetooth) but you can get one off eBay that will be compatible and make sure it is not blacklisted in your HP BIOS list.
Check out the OS X86 sites.There are several, some better and some friendlier than others. I found Prasys to be a good one. However, I only know about what I have personally experienced.
Windows of course just installs in its own partition, and will come up as an option when boot is initiated. If you install Macdrive in XP you can access all your OS X files as well from Windows.
You can obtain the Snow Leopard disk thru the usual channels.
For more info Google is your friend.