Sometimes one like -2147221499 or 80040005 and you have no clue what it is?
You can lookup error codes on Microsoft Website:
But that list is only for regular error codes.
The COM/OLE functions return different error codes called HRESULT. They can be decoded, see Structure of COM Error Codes and there are various lists for HRESULT values.
Which brought me last year to write a little tool to collect errors from various header files and put them in a big table:
Windows Error Codes
There you can quickly lookup the error codes, in both decimal and hex notation.