sfdisk: ERROR: sector 0 does not have an msdos signature

AMigos:

Tras instalar ubuntu GRUB no corre. Ya habia sucedido antes en esta misma maquina y se soluciono con:

# sfdisk -d /dev/hda | sfdisk --no-reread --force -H255 /dev/hda

EL problema es que ahora no funciona, aparece lo siguiente:

Disk /dev/hda: 155061 cylinders, 255 heads, 63 sectors/track
Warning: extended partition does not start at a cylinder boundary.
DOS and Linux will interpret the contents differently.
Old situation:
Warning: The partition table looks like it was made
for C/H/S=*/16/63 (instead of 155061/255/63).
For this listing I'll assume that geometry.
Units = cylinders of 516096 bytes, blocks of 1024 bytes, counting from 0

Device Boot Start End #cyls #blocks Id System
/dev/hda1 0+ 20320- 20321- 10241406 7 HPFS/NTFS
/dev/hda2 20320+ 82258 61939- 31217067+ f W95 Ext'd (LBA)
/dev/hda3 * 82259 101634 19376 9765504 83 Linux
/dev/hda4 101635 155060 53426 26926704 83 Linux
/dev/hda5 20320+ 81265- 60945- 30716248+ 7 HPFS/NTFS
/dev/hda6 81266+ 82258 993- 500440+ 82 Linux swap

sfdisk: unrecognized input: extended partition does not start at a cylinder boundary.

Estoy sumamente preocupado, espero que solo sea un error en la linea de comando.

Mi tabla de particiones es:

hda1 Windows.
hda3 Intalaciòn de ubuntu.
hda4 /home
hda6 swap

Porfavor ayuda!!!