There is 'daily price capping' on PAYG Oyster, so you should end up paying no more than a one day travelcard for the zones you travel in.
Having said that, I've seen others report that Oyster can get confused if you do the non-typical-passenger thing of travelling from A to B, not leaving the station, then back to A. If you have only touched in at A, and are then ticket checked while you're travelling back to A some time later, then questions might be asked, and it might be assumed you've not properly touched out / in at your remote destination.
Also, I may be stating the obvious here, but you mention going to Windsor - I trust you're aware that (with a few exceptions) Oyster is not valid, either in PAYG or Travelcard mode, at stations outside Greater London / the London fare zones (there are bits of the network that are in TfL zones that are outside administrative London)
The last stations within the London zones towards Windsor are Feltham (SWT) and West Drayton (FGW). I suspect if you turned up at Windsor (or had your ticket checked west of Feltham) and only had a PAYG Oyster - even if you had correctly touched in at Waterloo - then you may risk a penalty fare.
You can (before boarding, from a staffed ticket office) buy a "boundary zone 6" to anywhere you like ticket, but I'm not sure how this works in practice with PAYG Oyster - I suspect you might have to get off at Feltham and beep your Oyster card so it registers the journey from wherever to zone 6.
If you do it with a paper travelcard and a boundary zone 6 ticket, you don't need to get off the train at the boundary station and not sure you even need to travel on a train that stops there.
