File Overwrite - What Does Windows Do First?

I'm using Persits ASP Upload to upload files to a simple document
management application running on Windows Server 2003. At the moment
I've set the OVERWRITEFILES property to FALSE so every time a file is
updated ie the same file is uploaded, the filename is extended by a
postfix eg if I upload logfile.txt and then upload it again the new file
is called logfile(1).txt This is required because the code first uploads
the new file and then deletes the old one. The drawback is that the file
name extends and extends and extends...

My question is - if I set OVERWRITEFILES to TRUE and the upload fails
half way through, will the original file be trashed or will Windows
somehow 'roll back'?