Virtual memory is a mechanism Windows uses when programs need more memory
than you have installed. Basically, Windows uses space on your hard disk, in
something called the 'page file', and data is swapped back and forth between
RAM and the hard disk as needed.
The message means that, between your RAM and the page file, there is still
not enough memory to run all the programs you have open. Here are the most
likely causes (in no particular order):
1. Viruses and spyware.
2. Too many programs are open.
3. Insufficient RAM.