Run System Restore to before the changes, see if that helps.
Create a new User Acoount Paul with Admin privileges.
You can enable the real Administrator Account so you can see it and use in emergencies. It can be hidden again later if you want.
Switch User/Sign out/Log off to see choice of accounts
Hopefully HP is not the real Admin account renamed. Not even sure you can do that. It's probably bad practice anyway.
Did the seller give you a W7 Installation Recovery DVD (not just a Repair Disc)?
Or is there a Recovery Partition still on the main drive?
It's best to think ahead for worst case problems.
If System Restore fixes the HP Paul problems, and before you install lots of stuff you could make your own set of 3xRecovery DVDs.
Macrium Reflect Free would do that.