Unless there was a firmware update for your BIOS that added 'boot from USB', you will not be able to boot from USB.

Then Slackware is the best choice IMHO.

Upgrading to 1 GB, as suggested by btmiller, would be good if possible but I am not sure you will easily find fitting memory chips for a 10 years old computer. Anyhow 512 M is certainly enough to run Slackware.

Ubuntu and derivatives like Mint, or Fedora would offer more auto-configuration tools but that won't help you for hands on learning Linux.

There are other choices which you could consider like Arch or even LFS but it will be easier to begin with Slackware and try one of these when you will have learned the basis like editing configuration files, command line usage, basics of shell scripting or compiling yourself software.

By the way, there are very good sources of documentation about Linux @ http://tldp.org

Well, depending on the BIOS your motherboard has, you can generally select between various options for device boot priority. For instance, on mine you navigate to the "Boot Priority" tab, select a position on the list, and then select what device I want to have occupying that boot priority. If "USB Drive" or some other such option is on that list you can boot from it by putting it at a higher priority than anything else that the computer can successfully boot from. If no such option exists, then there is no way that somebody who doesn't already know how could do it. That is the same way you would select to boot from a DVD, by selecting the device you are putting the DVD in. My list generally has something like 'CD-ROM' at the top, followed by 'QUANTUM-FIREBALL-3.5"-HDD' (that is what my hard drive is called), or sometimes I put '3.5"Floppy Disk'.

Lean away from Slackware if you are a new user and towards Mint, Ubuntu, OpenSuse, Fedora, etc., unless you are a glutton for punishment.

The OP has specifically asked for a "hands on" distro, not a beginner friendly one. So recommending Slackware, Arch or Gentoo (or any other "hands on" distro) is the way to go, recommending the beginner friendly distros, regardless if the OP wants them, is not.

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I concur with Didier's recommendation for Slackware. It's not that hard, and once you get used to it, there is a refreshing transparency to it that will greatly increase your learning curve. At least it did for me.

If your system does not have the option to boot form USB, you can create a plop boot floppy or boot CD; http://www.plop.at/en/bootmanagers.html

It will pick up USB ports on the computer and allows you to boot from a live USB.

The only problem (in case of floppies) is that heads of floppy drives are not always aligned the same way. So a computer might not be able to reliably read a floppy created on another computer. I just went through that exercise for a system without CDrom drive. So CD/DVD is a safer bet in my opinion if you have that option.

That's a tough one to answer. If you can't find any options, concerning booting from usb when you dig around in your bios settings, you probably can't. Have a look around for any options regarding booting / devices / security in your bios.

I have had computers whose BIOS do not list usb as a boot option, yet I was able to boot from usb. On one such laptop, I remember in order to do it I had to press F12.