Big Problem: Songs Deleting from iTunes

[livetoberandom]

I use Limewire with a Mac OS X and I download a lot of songs. At one point it told me that my harddrive was running out of space, so I switched from Shared to a custom folder on my desktop for file storage. Soon, certain songs on ITunes started disappearing. I would see an icon to the left that looked like an exclaimation point and if I clicked on the song it would say "The song (insert name here) could not be used because the original file could not be found" and asks if I would like to locate it. I used to be able to find the songs somewhere else but now they seem to have been erased off the harddrive. It seems as if every time I download another song or movie, 5 or move randomized songs disappear. I'm really confused here: I'm about to get an iPod Video and I would hate for all my favorite songs to be gone. I've already had to delete a lot of songs because they apparently disappeared from the harddrive. PLEASE help!