Acid_1,
I wish I didn't have to almost always, it seems, give you bad news.
You have erased your partition table as well as your mbr. The dd command once executed is unrecoverable. As you have discovered, you can put grub back into the mbr pointing to the menu.lst file and the Linux kernel. That will get your Linux system running but you have nothing in your master partition table so there is nothing there for Linux to read. From a practical stand point of view, it will be rather difficult to straighten this out without loosing the system now on the drive.
You should be able to mount usb storage and have it recognized by Linux. Then transfer anything you want to keep to that storage device. If you have separate data partitions on that drive, you will probably loose all that data.
Then repartition and start over. I know that is not what you want to hear, but it is probably where you will wind up.
The next time you want to zero your mbr and not touch your master partition table, (mpt), use the dd command below:
dd if=/dev/zero of=/dev/sdb bs=
446 count=1
Look at the url below to see how that 512 byte sector is structured and you will understand why.
http://en.wikipedia.org/wiki/Master_boot_record
Fred