Can’t Defrag

I am using Win 98 SE on a PC (Intel Celeron 1.2GHz, 256Mb and an 80Gb hard drive), and when I try to defrag I get the following message:
Your computer does not have enough free memory to defragment this drive. Quit one or more programs, then try again.

I have closed everything in my system tray and turned offf my screen saver and also tried restarting in MSDOS mode and typing win /d:m and I still get the same message. Please can you help?