Mac OS X dynamically assigns memory space. As such if you open many applications, space can get filled up quickly. But the error could be erroneous because of some applications which aren't compatible. See my FAQ* on what may be filling your machine:
Note: PPC is only for Macs that were made before 2006. All Macs now use Intel. I've requested a moderator move this thread to a better forum. No action is needed on your part.
Have a look at how space is being used:
OmniDiskSweeper (free) - http://www.omnigroup.com/applications/omnidisksweeper/download/
OS X Lion: About Time Machine's "local snapshots" on portable Macs - http://support.apple.com/kb/HT4878 - “You may notice a difference in available space statistics between Disk Utility, Finder, and Get Info inspectors. This is expected and can be safely ignored. The Finder displays the available space on the disk without accounting for the local snapshots, because local snapshots will surrender their disk space if needed."
Hidden local backups with Mac OS X Lion filling up your MacBook hard disk! - http://blog.valerio.de/post/19238158977/hidden-local-backups-with-mac-os-x-lion- filling-up-your - Time Machine using local drive to store backups when external drive is not available.