It's an ancient backup format.
Nobody needs it, because you can get a copy of
Macrium Reflect Free, install it, and
back up your computer to an external
drive for safe keeping.
Because there are *multiple* free backup tools
for WinXP or later computers, we have absolutely
no need to dabble in the old backup format that
came with WinXP.
Most backup software, uses VSS service on the
computer (VSS is available on WinXP or later).
That allows backing up the C: partition, while
Windows is running (unlike the older Ghost software,
where you rebooted to do the backup). I think even the
built-in Windows Backup that came with WinXP
can do that too. I've only tested it once,
long enough to test a conversion software,
that could convert a .bkf file to another
format. That's the only time I played with the
ancient way.
So those spam messages, are selling a repair tool,
for a backup scheme that nobody should be using
any more. The thing is, some of the newer backup
tools, they have their own conversion and viewing
options, which are much more effective. For example,
on Macrium, the tool itself supports .mrimg to .vhd
conversion, as well as mounting any .mrimg file you
click as if it was a disk drive. This gives you
random access to the files inside the .mrimg.
Acronis also makes software with this capability.
Paul