Thanks for the reply Bungleau - I never thought about fragmentation - good point!
Alas, my quota settings are disabled
To add a little bit of information, it pops up with a balloon, similar to the USB speed warning for example. Clicking the balloon fires up the Drive Cleanup Wizard, but I can't find any options either there, or in disk properties.
Windows may be the easiest operating system to use in my experience, but it sure isn't the easiest one to administer!