In fact this change is the source of a number of forum posts akin to 'my fonts have gone bold, help!'. I myself spent a long time searching for a solution to this 'problem' - last time it occurred I ended up re creating my user account from scratch! I'm not sure why or how but this time it occurred to me to look up ClearType - I guess I saw it on one of the forums that discusses the issue.
I still have no idea what actually causes the change to happen but here's how to undo it:
- Open the control panel set called Personalization (right click on the desktop > Personalization)
- Click 'Window Color and Appearance' then 'Open classic appearance properties for more color options' which is at the bottom
- In the new dialogue box click 'Effects'
- In the next new dialogue box tick 'Use the following method to smooth edges of screen fonts' and set the drop-down to be 'ClearType'
- Click OK on both dialogue boxes to close them, applying the changes