Excel saves my workbook as another workbook that was open at the t

I am using Office 2000. Somtimes when I have multiple workbooks open and I
save one of them I get an error mesage (usually with larger i.e. 12Mb+ files)
saying "Document saved. Cannot re-open due to low memory".

When I re-open the file that I have saved, it has become one of the other
workbooks that I had open and the original file is lost! I used the "Save"
function not the "Save As" function so I can't have accidentally overwritten
the wrong file.

This has happened a couple of times, resulting in hours / days of lost work!
Can anybody advise why this happens and / or how to recover the original
file? It's driving me nuts!