Debian? sudo apt-get update && sudo apt-get distupgrade about 4 times? I took a machine from Ubuntu 7 to Ubuntu 13 that way with a few hiccups (mostly networking driver and netatalk related).
Just saying, it is at least doable, barring actual hardware-related reasons. With XP->7/8, it *should* be doable, but they made a decision to not support upgrading without deleting all of your files in the process.