For an explanantion of virtual memory see here
Virtual memory relies on disk space - you do not say what size disk you have - and if the disk is fairly full then this may be the problem.
Also it could be due to the amount of disk space allocated to virtual memory. Usually it is best left to the computer to organise this rather than specifying an amount. This can be done by going through My Computer/Control Panel/ System/Perfomance tab then the Virtual memory button. (Depends on what version of windows you are using but this is Ok up to ME)
Make sure that the button marked 'Let Windows manage my virtual memory settings' option is selected.
If you have also noticed a slowing down of the system when using obviously large files then it might be helpful to install more RAM or physical memory.