Out of memory, basically, means you're running out of memory.
Your machine has a limited capacity for RAM with which to carry out its functions you are doing.... when it reaches it's capacity, it can switch some of these RAM to your hard drive and keep them in virtual memory until you come to access them again.
You could try upgrading your RAM capacity, or also try freeing up some physical c: drive space (I get nervous if I have less than around 200 megs free) and notice serious performance degredation if it dips below 30 megs
Your filesize may be 20megs (that's pretty big, but by no means a total behemoth) but the functions and calculations it is carrying out may need 100 megs in any given millisecond of processing, so you may be trying to access your hardrive (do you hear lots of "whirring" ?) to compensate.
As Dreamboat suggested, try to slim your file sizes down to relieve your immediate RAM, but also try to free up some C: space.
Deleting MS Word is always a good start :wink: