If you find any bat files in Pen drive, CDs or receive any bat files through email,Please never open unless you are 100 sure that it does not contain malicious commands/scripts.To read what bat file contains,just right click on it and select edit.Then you can read the file.
A single line of script in Bat file may ruin your entire data and your system will collapse.They may delete the windows files, format data, steal information, consume CPU resources to affect performance, disable firewalls, open ports, modify or destroy registry and do many more losses.
In most of the cases,these bat files cannot be detected as Virus by the Anti Virus software. So these files may do harm for what they intended without Anti Virus notice.