That message often appears when the partition is not active. Try booting from the Windows DVD or a repair CD, choose the Repair option and then:
1. Choose Command Prompt and make the partition active by typing these commands:
DISKPART and press Enter.
LIST DISK and press Enter.
SELECT DISK N and press Enter (N represents the disk you want).
LIST PARTITION and press Enter.
SELECT PARTITION N and press Enter (N represents the partition you want).
ACTIVE and press Enter.
EXIT and press Enter.
2. If the above doesn't cure the problem, go to the Command Prompt, then type these 4 commands:
BOOTREC /FIXMBR and press Enter.
BOOTREC /FIXBOOT and press Enter.
BOOTREC /SCANOS and press Enter.
BOOTREC /REBUILDBCD and press Enter.
3. Lastly, go back to the Repair option and choose Startup Repair (you may have to run it several times).
The commands are in capitals for clarity only but note the spaces in them.
