Discovering an original parquet floor underneath an old wall-to-wall carpet is a great and valuable discovery. Specially if the floor is still in one piece, i.e. no rows or tiles removed for installing a central heating system or other 'modern' plumbing work.
(See also our Wood Floor Guide: "7 Easy Steps to Repair/Restore Your Parquet Floor")
Restoring the parquet floor to its original lustre is really a 'labour of love', but with the right tools and products it will regain its beauty for many years to come. First up is to remove all carpet residue, like the sticky (rubber) underlayment. Scrapping will get most off. If you need to use chemicals try it out first in a corner behind a door - ventilate the room sufficiently and READ the instruction before hand!
Many old parquet floors were stuck down with bitumen which over time can become very brittle and loose its bonding power. Loose blocks (rattling underneath your feet when you walk over it) could be the result. Most old wood blocks have small T&G's all around, lifting one of the block could result in a kind of cascading effect, more loose blocks. So be careful when trying to remove loose blocks.
Before you re-install the lifted blocks remove as much of the bitumen from block and underfloor as possible. Any large residue of the bitumen will make the floor uneven, plus the time the modern adhesive will take to fully bond with block and underfloor will be longer - in cases we've seen even days longer!
Some re-found original parquet floors only need a bit of extra TLC, remove all dirt and apply a suitable maintenance product.
Others however take more work, specially when blocks had to re-installed (or 'new' blocks found to fill in empty spaces), the wear and tear layer is rather damaged (by carpet grippers etc). Then sanding the whole floor is the only solution (such a shame to cover your valuable parquet floor with carpet again!).
Before you go out and hire the first sander you can find, a word (two words really) of advice: remove what's left of the old finish material and hire the proper sanding equipment.
No matter with what grit you start sanding, if your old floor still has layers upon layers of wax on and in it you'll spend a fortune on sanding sheets! Try out a tiny corner of the floor with a sanding paper by hand. If the paper fills right up and spreads rubbish (warmed wax) all over the floor you'll first have to remove this old wear and tear layer. Apply Wax and Polish remover - turpentine or white spirit based - on a cloth and remove the old layers as best as possible. Again, VENTILATE & READ the instructions on the tin/bottle.
For the best end result after sanding your original parquet flooring you'll have to use (hire) a professional belt-sander 
(and edge-sander). Like the description says a belt-sander has a continuous (or endless) belt, or rather continuous (or
endless) sanding paper - called Belt cloth. These belts are very easy to 'wrap' around the drum of the sander (1 minute tops), but most importantly will give the smoothest results on your floor.
Most DIY hire centres can only supply you with the (much lighter) drum-sander. Drum sanders have sheets of sanding paper, that has to be wrapped around the drum and fixed firmly in place with a metal bar.
This metal bar, when not attached correctly will create shatter marks all over your floor. The problem is that those shatter marks will mostly only show up AFTER you applied the finish product (varnish or HardWaxOil). Trying to remove these marks with a rotary sander will create another problem: circular marks in your floor.
So for a 'professional' restored original parquet floor use/hire professional tools.
More tips and advice on restoring existing Oak flooring see our Wood-Guide on the "Ageing Wood Concept" - retoring even the authentic patina of your Oak floor.