Out of Memory

I have an old Excel workbook with a lot of formulas. it's been running really slow, but it finally has locked up. I went in using safe mode, but most of hte sheets are hidden. when I was unhiding the sheets I get a popup saying out of memory. Is this a computer issue? Is this a memory leak? Any insight in what's going on? I'm trying to split the workbook into three different workbooks to get the size or whatever down, but I don't know how to do it if I don't have enough memory to have all the worksheets visible.