Microsoft ISA Firewall client may cause Fiddler to detach. When starting Fiddler under nonadmin account (ordinary User) you may see an error message:

Unable to bind to port [Localhost: 8888]. This is usually due to another running copy of Fiddler.
(An attempt was made to access a socket in a way forbidden by its access permissions)

Fix: Close Fiddler. Using REGEDIT, add a new STRING under HKCU\Software\Microsoft\Fiddler2 named ExclusivePort with value False