The neatest way would be to test the archive bit of the currentsong.txt file. If the attribute is set then the file has changed so ftp...
I can't find how to do that so here is a quick solution based on if the file exists.
1. wait 15 seconds
2. check if file exists
3. If yes ftp the file and then move the file (rename to something else)
4. goto 1
The main problem here now is the wait 15 seconds bit as there is no direct command in XP / DOS to do this. You will either have to:
download Windows Server 2003 Resource Kit tools (30Mb download just for one tiny app)
use a third party utils suite (may not be free)
Try a hack using ping. For this example I'll use the ping hack.
PING -n 16 127.0.0.1>nul
if exist c:\pathtofile\currentsong.txt goto file_updated
*ftp -b .... your ftp line here
move /Y c:\pathtofile\currentsong.txt c:\pathtofile\lastsong.txt
BTW, I would recommend downloading psftp from putty. This is a secure ftp which has better ftp batch processing commands. You would need to ensure that your server accepts ssh ports first.