Thankfully, Ubuntu 12.04 finally fixes the bug that caused earlier versions to trash valid EFI System Partitions (ESPs). Thus, you can install 12.04 (relatively) safely, without undue fear that it will trash your existing EFI boot loader(s). That said, backing up your ESP is a good idea before any OS installation. You can do this in Windows by opening an Administrator terminal and typing "mountvol S: /S". This will mount the ESP as S:, so you can back it up. (It's just files on a FAT filesystem; unlike BIOS, EFI doesn't rely on code hidden in boot sectors.)
To more directly answer the first part of your first question, I can't say whether you should restore from HP's disks or install Windows in some other way. That's up to you, really, and your choice has more to do with your preferences for what's in your Windows installation than with how Ubuntu will react to it.
The second part of your first question is tricky to answer. Ubuntu will boot up in either BIOS mode or in EFI mode, and which one depends on your firmware and the options it provides to control the boot process. Many EFI implementations provide very poor user control over this matter, so it may be a bit hit-or-miss. In a worst-case scenario, though, Ubuntu will install in BIOS mode and you'll be able to convert from that to EFI mode by switching the boot loader. You might be able to force an EFI-mode installation by installing my rEFInd boot manager and booting the Ubuntu installer via rEFInd. (You may want rEFInd or something similar to switch between Windows and Ubuntu once everything's running, so installing it now can be beneficial in the long run.) By default, rEFInd shows only EFI-bootable OSes, including optical disc installers, in its menu, so if you boot the Ubuntu installer via rEFInd, you can be pretty sure it's running in EFI mode. It will then install grub-efi rather than grub-pc, so you shouldn't need to deal with the boot mode after installing.
Your second question is a little unclear to me. You can certainly partition your disk ahead of time, using either the Master Boot Record (MBR) or GUID Partition Table (GPT) partitioning scheme. You can also create an ESP ahead of time if you like. (In fact, I recommend doing this rather than letting Ubuntu create the ESP; Ubuntu's default ESP is about 1/10 the size it should be! I recommend using an ESP in the 200-500 MiB range.)
