invoke CreateFile,[filename],READ_WRITE,SHARE,NULL,OPEN_ALWAYS,NULL
mov [fileh001],eax
invoke GetFileSize,[fileh001],NULL
mov [fsize],eax
invoke CreateFileMapping,[fileh001],NULL,PAGE_READWRITE,0,0,NULL
mov [file1h],eax
invoke MapViewOfFile,[file1h],FILE_MAP_WRITE,0,0,0
mov [file1],eax
    

catch the return values on every api call you use, and lookup error text (there some api for that too, GetErrorText or something like that). and do this always so you can find error easier.

you don't know windows. error can pop out anywhere, not only where it should. I used to have macros for error catching, it shortens code nicely

it is not easy figure out your problem clear to me, so:
0.read carefully about functions you are using and check well parameters you are passing to them. esp. check if you are using zero instead of pointer to empty asciiz string and pointer to var where written bytes number will be stored.
1.split your code into parts, comment them though it has valid prolog and epilog, then compile and run, and check if there an error appeared: eg. open file and immediatelly close it. if no error there - unkomment next part, etc. - with this lazy way you can catch where your program starts crashing even without debugger.
2.just use debugger - ollydebug shows description of most errors
3.if this will not help: i guess your project is not too huge - zip it whole and post here so we can check it (also include small example files you are going to xor)

regards!