>Hi,
>Does anyone know to calculate the reverse of a checksum that was generated with the SYS(2007) function? How are checksums calculated?
>
>Example: aa=val(sys(2007,"4848694856494948556965701264956495752505351565052")) / 15
>The calculated value of aa is 2840.9333
>
>Now, to authenticate, what do I do to calculate and get the value 15, when all I know is the 2840.9333 and the string of "4848694856494948556965701264956495752505351565052".

No it's not reversible. SYS(2007) always return 2 bytes integer regardless of the size of the string. Because of that the same checksum could be generated for different strings. There's an article at http://www.ciphersbyritter.com/ARTS/CRCMYST.HTM that discusses algorithm behind SYS(2007) in details.
For serious encripton take a look at http://fox.wikis.com/wc.dll?Wiki~FoxCrypto~VFP and Tap into Windows CryptoAPI Encryption file #9561.