I don't know if this will help anyone but I never had this problem until I installed VMware on my windows 7 , and then my n900 was not charging and displaying the 'not charging. insufficient power' message. so I knew it was something to do with VMware.

the way I fixed this problem was to disable the 'VMware USB Arbitration Service' by opening the msconfig tool and unticking the 'VMware USB Arbitration Service' under Services Tap,