Q189862: How To Do 64-bit Arithmetic in VBA

A number of Windows APIs require a 64-bit value as either a single 8-byte integer, or as two 4-byte integers. This article contains sample code the you can use to convert data between an 8-byte integer and two 4-byte integers. It also contains sample code that you can use to perform simple mathematical operations on the integers, such as would be required when you increment the file pointer in the SetFilePointer API.

http://support.microsoft.com/kb/189862