1. The Ubuntu installer should give you the option of shrinking the XP partition to make room for Ubuntu. Shrinking the XP partition shouldn't damage anything, but partition manipulation is inherently risky. Make backups of any critical files first. And you'd ideally have backups or restore discs for the Windows OS, too.
If the partition resizing goes well, there's little chance of Ubuntu later screwing up your Windows files.
2. A failed BIOS flash can "brick" your computer, rendering recovery very difficult. Installing Ubuntu can't do that much damage, but it could corrupt data on your hard drive, including the Windows install, rendering it unbootable.
If it gets really screwed up, you'd need to reinstall any OSs from discs. If just the bootloader gets screwed up, you can restore the original Windows bootloader without reinstalling the OS, ideally with a separate Windows install disc or repair disc and the "fixmbr" command.
Installing Ubuntu probably won't mess up your system, but there is a small chance it'll screw it up. (You'd also incur the same risks trying to install two versions of Windows on a single drive)
3. Do you have a 10 or a 10v? The graphics chip in the 10v has much better Linux support. So, I'd probably shy away from Linux on the 10.
But honestly, I don't see much point in dual-booting, unless you're a Linux user who wants to boot Windows for games.
If you're interested in learning Unix/Linux, I'd just create a full-backup image of the OEM Windows install, then install Linux as a single-boot system. And if you're not truly interested in learning Linux, I think you'll just find it frustrating, and you'll never boot it if you're dual-booting.
I hope this info helps you make a decision.