You could try to REPAIR the Operating System, by,
setting the BIOS to LOAD from the CD drive, first, of course the Operating System Install CD already in the CD/DVD player.
To Allow it to BOOT up from there, so you can do a REPAIR of the Operating System (OS) when you do a "SAVE and Exit" from the BIOS.
This action should 'OVERWRITE' that damaged Bootmgr' file for you., but, be warned, it WILL be like trying to FULLY install the whole OS all over again! It's not a 5 minute fix-up job!)
It works as effectively ...in using a "Repair Disc", anyway.
Which makes me ask '"how did the ''Bootmgr' file get damaged, in the first place?"
Only thing that can do that, is a computer virus!
What Anti-Virus program are you using (IF any at all)?
Would strongly suggest that you get one that DEFINITELY does cover the BOOT-up sequence (all the way into the BIOS).
AVG, from Grisoft, is one.
It does offer a 'free for personal use' version. (Microsoft accepts this one, too.)
Microsoft's own Securtiy Essentials is another, works very well. (If you know 'how' to really set it up right!) It also will keep your UPDATED at all times, via the Windows Upate program, if you don't update it yourself manually. On a regular basis, which
most people won't do!