Bootsect is not included in the installed version of 7 , tho it is on the Windows 7 DVD/Repair CD or winre you can boot from the F8 Repair my Computer option.
It is also included in Easybcd:
EasyBCD 2.0.2 - NeoSmart Technologies
In Easybcd , click "Useful Utilities" then Power Console ( the black box like command window), type:
bootsect /nt60 <whatever the 7 drive letter is>:
then press enter.
Mark 7 partition Active.
Restart and you're done.
Using DVD/Repair CD/WinRE Command Line,
mark 7 partition Active, go to command prompt, type:
Diskpart
lis vol
( get the 7 drive letter - it might be different from WinRe)
exit.
Then:
bootsect /nt60 <whatever the 7 drive letter is>:
press enter
Restart - that's it.