I have one Single working now under CGminer 2.4.2 and before 2.4.1 without any problems, but I use Windows Server 2003
Lucky you. 2.4.1 and 2.4.2 has the same error handling with regards to this issue.
FWIW:
I have a Win7/x64 rig with (1) 5870 and (5) BFL Singles running 2.4.1 for over two weeks without issue. None of them have dropped offline, all are running 832 fw, CGminer says 826Mh/s.