i dont know very much about C at all but try build a lib file,such as masm32.lib and include that in your C source, of course you will have to build a C header file from it, but that shouldn't be to much of a problem
Use :
.model small, c ; c calling convention
ChecksumFile PROTO Handle:WORD, pChecksum: WORD ; pointer size is 16-bit, you must use a 16-bit C compiler or it will not work