I've been in touch with Microsoft. Apparently the proliferation of the BAK files was caused by a virus or Trojan. I've agreed to pay $49, and I'm sending e-mails to a rep who has guided me through mysterious steps involving msconfig and other daunting terms.

I've also done two full scans and removed what they told me to remove.

I had 94 copies of some of those silly files!

However, I just deleted them all again, and they seem to be gone for good. With any luck, I will never see them again.

Just thought people would be interested to know...