Hi
My hubby has this trouble on his laptop, and from what I can make out it is:
If you are working at full capacity of your RAM and the progs need a little more, they borrow it from your hard drive (I think) so its virtual because its not really there in the first place. Thats as simple as I can work it out.
Who knows maybe I'm wrong, but I know it happens if hubby tries to run Paint Shop Pro at the same time as Photoshop CS2, so it sounds about right.
I'm sure someone will put us both right!
Anne
PS Now I'm confused as well!