Virtual Memory Alert Message

I am using Windows NT on HP Brio PIII/800MHZ with 128MB RAM, 20GB HDD. Virtual Memory is set to 131MB. Normally use Oracle based programs. At times system gives an alert i.e
“system is running low on virtual memory …..”
I tried to increase the virtual memory but NT does not accept. After this alert, running programs are closed.
How can I avoid this problem