If you want to really preserve the underlying value, you need to format the cells as text (for instance, zip codes). You can't do math with text (not easily, anyway) -- it's normally for ID's and zips, etc.
Hi and welcome to the board!!!
My good friend Smitty gave you the practicle answer. You can, however, use Custom Formatting. Format>Cells>Number Tab. Choose Custom for category and for type enter as many zeros as you want. (Ex 000000). This will display the number with leading zeros, and you don't have to type them in. Excel will figure it out. The advandage is the cell still contains a number for calculation purposes. With my above format, entering 7486 will display as 007486