unable to start PC [HP p6790],'Bootmgr' is missing,do not have discs

[middenmess]

As title.
Bought used desktop hp P6790 running Win 7 Home Premium,64bit. No discs.

Worked fine until I tried to install Window updates which resulted in black screen with only the cursor on it.

Tried to reboot in safe mode/safe mode with networking but after a brief scroll of drivers loading it stops with 'windows\system32\drivers\AVGIDSEH.SYS' AT BOTTOM OF PAGE and message 'Please wait.'

After some time it reverts back to the black screen with cursor.

Tried to download Win 7 from Microsoft site but product key is not recognised presumably as Win 7 was pre-installed by HP.

Unable to 'repair computer'

Windows failed to start A recent hardware or software change might be the cause

Status: 0xc000000f
Info: The boot selection failed because a required device is inaccessible

Any advice or help please?

 

[GokAy]

Try disable AVG - https://support.avg.com/SupportArticleView?l=en_US&urlname=Disabling-AVG-when-Windows-cannot-start