Missing operative system error. GRUB. Help needed

Hello,
Firstly I would like to say that I used google, forum, etc. Couldn't find any answer.

I had my backtrack 4 RC1 installed, then I've installed Windows 7 Ultimate. I can boot only Windows 7. Previously I could operate on backtrack. Heres my partition specification:

/dev/sda1 * 1 16 128488+ 83 Linux
/dev/sda2 17 12240 98189280 5 Extended
/dev/sda3 12241 12253 102400 7 HPFS/NTFS
/dev/sda4 12253 38914 214149120 7 HPFS/NTFS
/dev/sda5 17 82 530113+ 82 Linux swap / Solaris
/dev/sda6 83 12240 97659103+ 83 Linux

/dev/sda1 is location of /boot folder and I've set it to be active partition.

I recieve error "MISSING OPERATIVE SYSTEM" when try to boot (get boot menu).

My GRUB menu.lst is

splashimage=070590ad-4e09-47b7-a398-38a8a77a332b/grub/splash.xpm.gz
timeout 8

title BackTrack 4, kernel 2.6.34
uuid 070590ad-4e09-47b7-a398-38a8a77a332b
kernel /vmlinuz-2.6.34 root=UUID=51b94169-b3bd-403b-812d-16a57d4e7fdc ro quiet splash
initrd /initrd.img-2.6.34
quiet

title BackTrack 4 (recovery mode)
uuid 070590ad-4e09-47b7-a398-38a8a77a332b
kernel /vmlinuz-2.6.34 root=UUID=51b94169-b3bd-403b-812d-16a57d4e7fdc ro single
initrd /initrd.img-2.6.34

title BackTrack 4, memtest86+
uuid 070590ad-4e09-47b7-a398-38a8a77a332b
kernel /memtest86+.bin
quiet


#windows 7
title Windows 7
root (hd0,3)
savedefault
makeactive
chainloader +1

I've manually added Windows 7 but it can't see any operating system.
What am I missing?
Please help,
Lowermind