Personal.xls is located in the XLStart directory, and is used to store
macros and
things that you want to be available to all workbooks, whenever you
start
Excel.

By default, it is hidden in Excel, but if you go to the VBE, you use it
as you can
with any other open  workbook (you don't have to unhide it).

You can create it by
- goto Tools>Macros>Record New Macro...
- Choose Personal Macro Workbook form the dropdown
- OK
- click the Stop button on the toolbar that pops-up

You now have a Personal.xls workbook. It is not visible though, it is
hidden
by default (Windows>Unhide)