Fiddler crashes on startup complaining about the Tahoma font

Sorry, you may have found a bug...

Fiddler has encountered an unexpected problem. 

Font 'Tahoma' does not support style 'Regular'.
Source: System.Drawing
at System.Drawing.Font.CreateNativeFont()

This can happen if you have the Microsoft Word 97 viewer installed. That tool sets the registry key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Fonts\Tahoma (TrueType) to tahoma.FOT. 
To fix the issue, change the following registry key from:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Fonts
"Tahoma (TrueType)"="tahoma.FOT"

to

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Fonts
"Tahoma (TrueType)"="TAHOMA.TTF"

If that does not help, you may be missing the Tahoma font on your computer (it should be in c:\windows\fonts), or the .NET Framework installation is corrupt. If you see the Font file, then try reinstalling the .NET Framework and all updates from WindowsUpdate.