Hah, I knew Quik would say to be wary of Lilo.
I've installed Slackware at least 20 times on different computers, and I haven't once had a problem with Lilo (ok, once -- but that was the computer's fault, the harddrive was set to slave and a master, or vice versa).
Partitioning your harddrive is easy, if you're willing to format it. You'll want 2 partitions for Slackware -- one should be double your ram (this will be used for Swap) and the other should be whatever you have left. You can do your partition tables before installing Slackware by running the command "cfdisk". It's pretty straight forward to use, just don't forget to "write".
If you need to preserve what's on the harddrive, that's trickier. Partition Magic can resize partitions, but it always seems to end up with me losing everything when I try, so I don't recommend it.
When you install Lilo, it'll let you add Windows and Linux partitions. I would recommend choosing the "Expert" option, then, in the main menu, do "Begin" (follow prompts -- when asked, chose to install it in the MBR) "Linux" (follow prompts) "Windows" (follow prompts) "Install". While Slackware installs, you automatically get to configure LILO -- it's the thing that asks you if you want to do simple/expert.
IF something horrible happens, Windows XP's recovery console can restore your MBR. You'll probably want to do a search for this, but boot off Windows XP's cd, go into the recovery console, and the command is SOMETHING LIKE "restorembr" or "fixmbr" -- look it up. If anything horrible happens (as Quik is convinced that it always will), you can do that to fix everything and have Windows booting itself again.
And yeah, Darkside is 266mhz, Slackware runs fine on it.
Any questions, feel free to post here.