Can't give any advices without more detailed description.
1. What the application that crashes with this error - L2 or L2Project?
2. What the state of current processes in Task Manager?
3. (Stupid question) May be your hard drive ran out of space?
4. Did you enable swap file in Windows? If no then you should enable it
5. Turn off memory-leakers in client like animations etc?
6. How often this happens? If it happens always then for how long the appl had been working before it crashed?