Lost my ability to boot Windoze

[David C. Hoos, Sr.]

Hi,

I have a Compaq laptop (Presario 1825 w/ 12Gb. disk) on which I
have been running Mandrake 7.1, and Windows 98 for some months.

All of a sudden, when I select Windows from the Grub menu, I get
the message Invalid system disk, but Linux still boots OK.

The Windows files seem to all still be there via /mnt/win.

Is there some way I can rewrite a sector, or some other way to
restore the booting capability without re-installing Windows?

Thanks for any help or enlightenment on the subject.