I just had the opportunity to try this. I had a Windows 7 system that I needed to sent to my IT department to wipe. So before sending it, I tried deleting the registry.
It isn't as easy as I thought.
First: You cannot delete the keys at the "HKEY" level, so you have go to the next level where there are hundreds and hundreds of keys to delete.
Second: Even after taking ownership and full access to all keys, I still could not delete every key. I could delete about 50% of those under HKEY_CLASSES_ROOT; all but Software under HKEY_CURRENT_USER (although "Keyboard Layout" kept reappearing); and nothing (or, at least, no complete key) under HKEY_LOCAL_MACHINE. However, when I got to HKEY LOCAL_MACHINE\SYSTEM, things started to fall apart. I kept getting a message box, "LicenseManRes.dll could not be found." and the registry editor hung. Control-Alt-Delete gave me an error message.
Cycling power put me into "Startup Repair" that gave me the option of restoring to a earlier point in time. I selected "Restore".
Ultimately it issued the message "Startup Repair cannot repair this computer automatically". It gave me the option to send information about this problem, and a few selections later it gave me the option to restart - which I took.
This started up "Startup Repair" - so I canceled and recycled power. The only options were Start Repair or open Windows normally - both put me into Startup Repair. There was no option to enter SAFE mode.
So I restarted with F8 and selected SAFE mode with Command Prompt. This got as far as loading CI.dll (not very far) and then dropped back into "Startup Repair". I allowed this to run one more time - and it failed.