There are a few DOS commands that should never be entered while Windows is running. Entering these DOS commands while Windows is running may cause data loss or corruption:
CHKDSK, SUBST, JOIN, FASTOPEN, or ASSIGN.
In addition to these commands, there are applications which may cause file loss or corruption, and should not be used when Windows is running:
Norton Utilities, Disk Doctor, or any other application that modifies the hard disk's File Allocation Table (FAT).