Disk space disappearing
My guess is you have a stuck process that is impairing the ability of the OS to tell you what amount of space is actually used from Applications, audio-movies-photo rather than categorizing everything as "other". 


Make a back up of the Mac in case it's corruption or a serious problem. 
Power off the Mac. 
Boot to safe mode - hold the shift key as soon as you hear the startup chime and release it 20 seconds after the grey screen is drawn. 


http://support.apple.com/kb/HT1564



That should free up a stuck process and also repair any disk corruption at is easily fixable. Please let us know if you continue to get bad information from the space allocation image. Surely Applications take up more than zero K.