Ich hab’ mal im Internet herumgestöbert.
Laut der Seite http://bsplanet.brokensword3.net ist die Lösung (Achtung, Englisch):
Under XP the game reports insufficient disk space when I have plenty.
This problem is easy to solve. Put the first CD ROM in your drive. Search for the files in the CD, and do right click on setup.exe file. At the compatibility menu tick the box to allow compatibilities and select Windows 95. Accept, and procede to install. You’ll install normally the game, and as you finish, go to the folder Sword where the game is installed (usually at C:\SWORD), and repeat the same procedure as with the setup.exe file, now with winsword.exe file. Now you’re able to run the game with no problems!
Tja - das war’s.
Viel Erfolg & Spaß beim Spielen.