Everything is almost certainly still there, just hidden.
Can you boot to Safe mode? (tap F8 repeatedly while booting)
Download and run Malwarebytes
All the major AntiVirus vendors now have boot/rescue discs.
If you can't boot to Safe Mode, can you burn one on another computer and try?
You don't need to fit a new HDD unless you have one knocking around. Re-installing everything is a pain and should only be done if you think cleaning the old HDD is impossible/too time consuming.
If you come across the exact name of the infection, let us know. Specific removal guides are available.
Post back before doing anything drastic. Do you have lots of stuff you don't want to lose or is it backed up?