Laptop won't boot; says "BOOTMGR is missing" after active partition was changed

I have a "Recovery" partition which I mistakenly thought was redundant after reinstalling everything to C:. "Recovery" was previously the "Active" partition. I set C: as the "Active" directory in disk manager (I am using Windows 7). When attempting to boot, the laptop now returns "BOOTMGR is missing".

I can go to BIOS and mess around with some stuff, but haven't found a way to change the active partition. I can disable various SATA drives (four are listed) and doing that sequentially changes the error message on booting, but no combination lets it boot.

I am travelling and don't have a USB key or bootable CD with me. I do have an external HD, but this other computer that I'm on right now (which is unusably slow) doesn't recognise it.

I think that the easiest solution will be to get hold of a USB key, make it bootable, and sort out the active partition from DOS. Any glaring shortcuts, alternative solutions or likely obstacles I'm missing?

Edit: I now have a USB key, can boot to DOS and run fdisk, which I expected to enable the active partition to be set. Unfortunately fdisk will not set NTFS partitions as active, and I haven't found any alternatives that run from DOS and will set NTFS partitions as active. At this stage it looks as though I will need to get Windows CDs as Olivier mentioned below.

The active partition is the partition which the system will use to boot from. In your case, you've changed it so that the required bootmgr could not be loaded. You need to set the partition named 'Reserved by system' to active. This partition usually has no letter assigned to it. You can set this with a bootable live USB running a partitioning program. Below I've placed a link which describes how to make such a live USB running easeUS:

http://www.partition-tool.com/resource/manage-partition/usb-partition-manager.htm

After you do this, everything should work again.

In the end I solved this by creating a bootable USB with the Windows ISO on it, and ran the "Repair" tool twice.

I essentially followed the instructions at: http://en.community.dell.com/support-forums/software-os/w/microsoft_os/3316.2-1-microsoft-windows-7-official-iso-download-links-digital-river.aspx

• In my case, I was using Windows 7 Professional (x86), so I downloaded that ISO.
• Then I used the "Microsoft USB/DVD download tool" to create the bootable USB.

Many thanks for the help.