I am not sure kjn2 understands what virtual memory is (or he is keeping his answer very simple), but the answer from redmug100 is correct.
A computer has real memory (or RAM). This is what is meant when it say 256Mb or 512Mb or 1Gb.
To run any program on a computer it has to be loaded into RAM.
When you start up your computer then programs are loaded in to this RAM (Windows is loaded in there for example).
Each program you start it is loaded into RAM (a word processor, each game, a web browser and so on).
But you only have so much RAM, so if it get filled up there is no room for any other programs (or parts of programs).
So what Windows does is create an area on your hard disk which is called Virtual Memory.
When your REAL memory gets filled up then Windows copies programs that are in Real memory into your VIRTUAL memory (on hard disk).
When Windows needs the program again it copies it from VIRTUAL memory back into REAL memory.
So Windows can be constantly copying programs from real memory to virtual memory and back again. This can slow your PC down.
Getting virtual memory errors probably means you do not have enough RAM (or real memory). Look to add more memory to your PC.
So if you have 256Mb then take it up to 512Mb or more. If you already have 512Mb take it up to 1Gb.
As redmug100 says, it could also mean there is not enough space on your hard disk for Windows to create the virtual memory.
Have a look at how much space you have on your hard disk and if it is low see if you can delete some file to create more space.
But I would say that you probably need more RAM (real memory) as this will cut down the need for so much virtual memory.