I had a similar problem with a N73 after I updated PC Suite from nokia.co.uk (to version 6.83.14.1). It seemed to work OK for a while, but some time after, lost the ability to connect (got the error message "there is no available connection type") when using the USB data cable. It looked like the USB dirver was corrupt or not starting because the N73 device didn't appear under the toolbar icon Unplug or Eject Hrdware.
After trying all the usual things (reinstall, reboot PC & phone) etc), I finally used Windows Uninstall to take off all Nokia programs (there were about 4 different things listed), and then used Windows Search to delete EVERYTHING to do with Nokia from the PC. That took out all the USB drivers etc, even stuff in the temporary files & folders. Everything. Kill it all.
Then download and re-install PC Suite.
That did it for me - happily re-connected