Can not defrag my computer

I have a PIII 850MHz computer with 1534MB of memory and 2- 45 gig hard drives and I’m running windows 98SE. Every time I try to run a defrag on my computer it starts then stiops and says to colse some programs because ther isn’t enough memory – the only things I have left open after contol-alt-deleting is the systray & explorer. How can I fix this so I can defrag? Thank you.