After running some basic recovery tools, I can now get Windows XP to boot normally in a usable (albeit slower and error prone) state. I still can't decide whether to try and apply a quick fix to the firewall and antivirus not being recognized or to perform a clean/repair install altogether (could be that some entries in the registry got corrupted during the interrupted shut down). For now, I plan to use XP as a secondary, offline OS.
I've also decided to try out one of the many Linux distributions available. After backing up all important files to a new external HDD, burning and trying out a couple of live DVDs and reading up on tutorials on how to manage HDD partitions to prepare for installation, I was able to get the laptop to dual boot into XP and Linux Mint.
Right now, I've installed the Cinnamon edition of Mint 17. It runs really well so far, but I'd like to spend more time to properly evaluate it. Some highlights would be that it was able to load my monitor calibration settings right away and it has support for Wacom tablets out-of-the-box. Though I still have concerns about other things like whether this new OS causes the laptop's components to heat up more than XP.
Well in any case, I'm due for an upgrade sooner or later since I've been using this laptop for around 4-5 years. Still, I prefer to replace things only when necessary, so I'll see if I can still make things work.
Also, it seems that it's just one hardware failure after another. Now the Xbox 360 just went red dot on me. At least I can continue to play the current 360 game I was preoccupied with since I also got it for the Vita (Blazblue CS:EX). I'm also gonna have to see if anything can be done to fix it, but I'll probably end up having to get a replacement. I'm mostly playing on portables these days anyway, so this is something that can wait.