File IO question

Question about file IO...
The basic scenario is that process A creates a file, writes it out, and
closes the file. Process B immediately opens the same file and reads
it. Is it possible that Windows hasn't fully written out the file when
process B opens it? Having process A flush the file out to disk before
closing it wouldn't be a good option for us, because it would severely
hamper the performance - a lot of file IO is going on. So, is there a
way to get a guarantee from Windows that the file exists in its entirety
- using file locking, perhaps?

-Mark-