I found that if I copy and paste a large amount of data, and there are hidden columns, it causes Excel to use all the memory in my system until it crashes. I originally did this with Excel 2007 Pro on a Windows 7 Laptop with 4GB RAM, and then I did it on a server with Dual Xeon X5560's with 24GB RAM and had the same results with Excel 2010 64-bit. If I unhide the columns it uses a few hundred megabyts of RAM. I am copying and pasting about 300000 rows with about 15 columns of mixed datatypes. (None bigger than about 30 characters)