Microsoft provides direct links to download official Windows 7 Service Pack 1 (SP1) through their website. While this doesn't directly download Bootmgr, installing SP1 can often repair or update Boot Manager files.
You do not need a product key to repair an existing installation. Microsoft provides a tool to download a legitimate ISO.
There are several reasons why you might need to download or repair the Windows 7 Boot Manager: windows 7 bootmgr download
If Startup Repair cannot fix the error, you will manually copy the file from the recovery media to your hard drive.
Step 1: Boot from the Windows 7 DVD/USB again.
Step 2: On the language screen, press Shift + F10 to open a Command Prompt.
Step 3: Identify your drives. Type diskpart then list volume. Note the letter of your DVD drive (usually D: or E:) and your Windows 7 hard drive (usually C:).
Step 4: Exit DiskPart by typing exit.
Step 5: In the command prompt, type the following commands (one by one, pressing Enter after each): Microsoft provides direct links to download official Windows
D:
cd boot
bootsect /nt60 SYS /mbr
This rewrites the master boot record compatible with BOOTMGR.
Step 6: Now copy the BOOTMGR file directly. Assuming your DVD drive is D: and your Windows drive is C:: This rewrites the master boot record compatible with BOOTMGR
copy D:\bootmgr C:\
Step 7: Reboot by typing wpeutil reboot.
If the file was simply missing, your PC will now boot. If not, the BCD is likely corrupt (see Method 3).