This is one of those errors which are really pain in the tooth. I encountered it yesterday and it ate up lots of my hours. To add insult to injury, the damned error appeared on the PC of department head. So I coudnt evade or postponed it.After trying virtually everything in my repertoire regarding Oracle net configuration debugging, I re-installed the software, but the error persisted. Then I started to look outside Oracle. I again pondered upon the wording of error and it was implying something wrong with network packet transportation. Just, beleive me, just through pure intuition, I thought perhaps the packets were blocking due to some checks and barriers. The most obvious barriers to the machines are antivirus programs. Zone Alarm antivirus was there. I uninstalled it and guess what? Yes it worked this time.