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)
Using REGEDIT, add a new STRING under HKCU\Software\Microsoft\Fiddler2 named ExclusivePort with value False