I fell fowl of Microsoft Knowledgebase 269826 by backing up to a .bkf
file of >4GB on a FAT32 drive from a Win2k NTFS machine.
I then lost the originals by reformatting my Win2k machine and have
now lost my last 3 years work!
If i try to restore I get the error message: 
"The backup file is unusable. You must erase it, or choose another
file. "
The backup consists of a 'normal' backup and then a small incremental
backup that pushed it over the 4GB edge. I did get an error at the
time but I thought i related to one particular .pst file which I then
separately backed up (and restored successfully).
Can I perhaps strip out the 2nd backup to reduce the file size and
restore it?
Any other possible methods? 
Regards 
Dave

First Of all I must state i have no direct experiance of the said
technet microsoft article. how ever since u have already done some
research on it.and know the cause  I should say that u could try
restoreing the backup to a NTFS partition(as the related technet
article states the cause of the error is the  limit of FAT and FAT32
of maximum file size 4 GB).To do that I suppose u have to install
windows useing a CD and choose NTFS as the format of the file system..
IF not
since its incremental. u can leave out the last few ones and try it.
after all missing out a few days work is better than missing out three
years. Good luck  :)

Thanks for the comment.
I have already tried copying the .bkf file to my Win2k NTFS formatted
PC but I still get the same restore error (unusable file).
Regards

I had a similar problem and fixed it using BKFRecovery.