Why - when my library is full on one day - do i come back the next and half the songs are gone?

Could you please provide your specs. It's really hard to get the slightest idea of what's happeneing without them?

Did you just file them away in the wrong place on accident perhaps? LW and it's uninstaller never delete any files created after installation.

are you possibly talking about the 'incomplete' folder?

AFAIK, limewire keeps files in there only for a certain number of days. you can set that number in the options under 'saving'.