hmm strange
after uninstalling the clinet, did you verify that no omniback folder or registry key was left?
i suggest you to uninstall client agent again, check that no omniback folder is left. if it is, delete it. check that serivce is removed.
reinstall agent again. if you install it localy then start setup with "run as administrator"
try disable antivirus if it is present on the client, maybe it causes troubles