So, what are the real advantages of running a Linux distro such as Slackware? I mean, if it was better than Windows, everybody and their brother would be using Linux, but they're not.
Porsches are better than VW's, but I see more people driving VW's than Porshes. You probably own some devices running Linux right now and you don't even know it (ditto for Java - which runs in over 1.5 billion devices according to Sun).
What kinds of things can Slackware or a similar distro do that Win XP Pro can't?
Does it run faster? Boot faster?
A large variety of free software, a kernel that you can configure to suit your needs, runs on non-x86 hardware, several interesting window managers, several filesystems, etc. Linux also provides people who are really interested in how a computer runs a way to tinker and try new things out.
If you think boot time is a meaningful OS criteria, you've probably wandered into the wrong place. No one developing Linux gives a rat's ass about how fast the system boots - and my monitor 'wakes up' much faster than your computer boots. Stability, security, good modular code, multi-platform support, and source code availability are the criteria Linux developers consider important. You'll also need to fight the urge to hit the reset button - only Windows users have this strange desire to reboot.
How much of my day to day activites can I perform on Linux? IE, scanning, photo editing (I have heard of GIMP), web surfing, DVD burning, CD burning, Word Processing, Internet Radio listening, MP3 playing, etc.
None of your Windows software is going to run natively in Linux. You might find that Wine is able to run some of it - this can be a real pita and IMO is simply not worth the effort. However, you will find native Linux alternatives for all the items you listed. You'll have to check on the availability of drivers for scanner as well.
It sounds like you might want to try Knoppix or the Slack cdroms. These are always a good option for getting your feet wet.