How does the checksum stored in the reserved interrupt vector get there? If you look at the binary file it contains the hex bytes for a NOP instruction. Once flashed the value stored in memory is now the correct 2's complement of the other vectors so it seems to be set somewhere in the loading process.
Did you try to search on this forum?
The same question was answered just two days ago: http://www.keil.com/forum/docs/thread12477.asp