Your 'tech' is not very good at his job and probably is trying to con you.
If he's swapped the hard drive, as he says, then he would have had to install a new copy of Windows on it.
It's impossible for there to be any corrupted files on it, unless he cloned the old drive to the new one. That would be a pointless exercise, because he'll be copying a defective system to a new drive!
AVG wouldn't find corrupted registry files, its only purpose is to scan for and remove malware/virii.
I suspect he hasn't installed a new drive at all, but is looking to get paid twice for 'fixing' it. Once for the 'new drive' and again for correcting the imagined corrupt files.
You can do yourself for free...
The key words here are SAFE MODE with Networking!!!
Download, (if you can't get on-line, have a friend download and copy to disc/flash for transfer) install, update and then in SAFE MODE* run full scans with these programs...
**Note: Malwarebytes should be run in 'Normal Mode' unless that isn't possible!
*** Important, when downloaded/installed/updated disconnect from the internet ***
If you can't run a scan, go into the scanner's folder and re-name the .exe file to something like 'MyFile.exe' and try in safe mode again. Some malware prevent the execution of anti spy/mal/virus scans by blocking them by name.
Try running a couple of on-line scans to be sure...
You should consider getting a good Anti-Virus program.
Try one of these free ones...
***Never use more than one, active, anti-virus program at any one time***
*Note: On start up (before Windows loads) keep tapping either F5 or F8 (be aware that some manufacturers use F8 for system recovery!) then use arrow keys to highlight 'Safe Mode with networking' and hit Enter/Return, click on an Administrator account, enter the password (if you don't know it, there probably isn't one so leave it blank) and hit Enter/Return.