Virtual memory problem, what should I do?

Hi all,
I have a desktop PC. I have installed NFS Most Wanted game on my PC, but when I play it, approximately after 25 minutes the game closes automatically and my PC displays a note that 'Your System's Virtual Memory is Low'. I don’t know why it is happening to my PC. As far I know that my PC covers all the requirement of that game.
Moreover, this problem also happened at the time of working on Ulead Video studio10. Really, I am so much irritated by this problem. Can anybody provide me a solution of this virtual memory problem? I will be very thankful to him. My PC configuration is below:
Intel Celeron D.
2.13 GHz processor,
256 MB RAM,
64 MB Graphics Card,
40 gigabyte Hard Disk.
