system restore error

I have a trojan virus that McAfee cannot completely remove and it has
affected system restore. I was attempting to turn off system restore to do a
scan in DOS mode, but I received a message "system restore encountered an
error trying to enable/disable one or more drives. restart computer and try
again." I restarted but the same happened. I am not able to use system
restore and I believe the virus is causing the problem. Any thoughts?
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JS