Startup disk doesn't have enough memory for applications

I keep getting this message that my startup disk doesn't have enough memory for applications, and advising me to quit apps. Even after a restart I get the same message. I am using only about 10% of my hard drive and I have 4GB or RAM. I have run Disk Utility to repair permissions, I have confirmed that my MacintoshHD is the startup drive, but still keep getting this message. I am running Snow Leopard on a three-year old iMac. What gives?