Cannot save updates of long Word documents

[bertietheblue]

I often work on large Word 2010 documents (200-500pp, upto 2MB), highlighting text. The more I highlight, the more saves I make, the slower my computer gets until finally I get this pop-up message:

'Word has insufficient memory. Do you want to save as Rescued document 1'



I save as Rescued document 1 and continue, but if I try saving again, I get the message:

'The save failed due to out of memory or disk space'

And if I close the Rescued document and try to reopen it, I get the message:

'Microsoft Office cannot open this file because it is an unsupported file type'

Is my computer the problem? My OS (Windows 7)? Any other ideas?

Thanks in advance for any advice
Bertie