Last night I began my quest to get rid of "OS Rot" on both my PowerBook and my Windows box. I bought new hard drives for both, so I wouldn't have to worry about losing any data. For the Mac, I bought a Lacie d2 (250GB) and for Windows, I bought an internal Maxtor 120GB. Thanks to everyone who suggested the Lacie.
To start, I cloned my PowerBook's drive to the Lacie drive using the free version of SuperDuper. It took about two hours and worked flawlessly. I then proceeded to format the PowerBook drive and install OS X. For the most part, I just copied a bunch of files back into place. I've been trying to restore my settings by copying individual folders from ~/Library to the fresh install - but it's not working so well. I'm thinking of just restoring my whole home directory (cruft in ~/Library and all).
The Windows install wasn't nearly as easy. Rather than backing up to an empty drive, I just installed the new disk as the primary and old one as a slave. I tried installing Windows on the new one twice (once w/ the slave installed, once w/o). After installing, when I boot up, it just sits there will a dark grey screen. So I gave up and put my old hard drive in as the primary. I think the disk might be bad. Regardless, I'm going to try again tonight. This time I'm going to use a ghosting/cloning program to backup to the new hard drive - and essentially go through the same steps I did on the Mac. I'll probably use Norton Ghost or PartitionMagic - but I'm open to other suggestions.