windows - Terrible font rendering in some applications but not others

I'm not sure what's changed, but all Windows updates are done, and all browsers are fully up to date. Chrome 40, Firefox 35, Opera 27.

This is what my fonts look like at the moment:

I have ClearType turned on, and if I open up Word and write some large type in the same font faces everything looks fine.

I tried changing the website I'm working on to Calibri, and that was still blocky and horrible even though it's a local font that I definitely have, and which Word displays correctly.

I've tried turning it off and on again. This is a recent problem (i.e., something has changed, it is not a problem associated with an initial installation).

Answer

Let's try to build an answer with the full information here...

This is a known issue with Microsoft Update KB3013455, released on the 10th February 2015.

For a quick way to check if you have the update installed, open a DOS command prompt and type:

wmic qfe list brief /format:texttablewsys | find "KB3013455"

If this returns no text at all, the update is not installed; if it is installed, you'll see KB3013455 and more text printed out.

The issue can be resolved by either

1. 
   

   uninstalling the update (but this will leave you with a security vulnerability). A reboot will be required.

   
   


2. 
   

   Installing a subsequent update (the Microsoft official fix, released on the 18th Feb 2015):

   
   

   
   

   To resolve this issue, install update 3037639. For more information, click the following article number to view the article in the Microsoft Knowledge Base:

   
   

   3037639 Fix for text quality degradation after security update 3013455 (MS15-010) is installed.

   
   

   
   

   This fix will take you to a page where it seems you're downloading the original KB3013455 all over again. Keep going, when you actually click to download you will get an option to get the fix, KB3037639.

   
   

   A reboot will be required.

   
   

   The fix was later included in Microsoft's automatic updates in March 2015.

   
   

Edit the list below to add affected systems:

• Windows Vista SP2


• Windows Server 2003 SP2


• Windows Server 2008 SP2


• Windows XP (for people on the Custom Support Program that keep receiving patches for XP)


• Windows FLP


• Windows Embedded (WEPOS, POS Ready 2009)

Edit the list below to add affected Fonts (only some font sizes show problems):

• Arial


• Courier New


• Times New Roman


• many others

Description of the problem (from here):

Essentially the small fonts (8-20pt) that normally result in a one-pixel line (think of lowercase "b" or "t" or "d") would now be a random mess between 1 and 2 pixels. Turning on "Clear Type" or font smoothing does "smooth" the garbage and it doesn't look as awful but that doesn't make the text any more readable.