Client Backups Fail Every Time

Hi. Having successfully installed WHS2011 I then set about trying to backup a client computer. I installed the connector which installed fine but when I try and do a manual backup it fails every time. It starts and stays at 1% progress for some minutes and then fails with the following error..An error prevented backup from running. An error prevented backup from creating a snapshot of the volumes on this computer, for more info see the event logs..

When I look in the event viewer source Server it says.
Backup job 3 on SERVER did not succeed. Reason: ClientVssProblem, System.String[]

When I look in the event viewer source VSS it says

Volume Shadow Copy Service information: The COM Server with CLSID {0b5a2c52-3eb9-470a-96e2-6c6d4570e40f} and name VssSnapshotMgmt cannot be started. [0x80070005, Access is denied.]

and also

Volume Shadow Copy Service error: Unexpected error querying for the IVssWriterCallback interface. hr = 0x80070005, Access is denied.

. This is often caused by incorrect security settings in either the writer or requestor process.

Operation:Gathering Writer Data

Context:
Writer Class Id: {e8132975-6f93-4464-a53e-1050253ae220}
Writer Name: System Writer
Writer Instance ID: {a89557b1-d3eb-4a5b-97df-3dcbd4d41939}

I notice from other posts that this problem can be caused by previously having 3rd party backup programs installed, I did have Comodo backup installed which I have since removed and cleaned the registry but other posts seem to suggest the ultimate fix is a clean install of the win 7 client PC which I am loathe to do. Has anyone got any suggestions ?
Can anyone help remedy this issue?

Thanks