I have an XP computer with lots of hard drive space partitioned separately for a <future> Linux install. the future has arrived! How can I do this? I would prefer not to burn a CD, but rather, just install from within XP.
Thanks.

Just burn the disc.
Doing this in in XP would require something like VMWare or Xen (and others) to run both systems (Windows and Linux) at the same time. This will not work optimal and isn't really for starters.

Just do a dual boot (at boot time, a boot menu will ask you if you wish to boot Windows or Linux).

Leave (or make) the hard disc space you wish to use for Linux as "free space".
Next pick and download an easy distro to start(Ubuntu, Fedora, SUSE, ...) and burn it on CD/DVD(s). The installer will pick up the free space(pick this option if available) and set up everything for you(including partitioning, configuring the boot loader for a dual boot, ...).