Please help me with Zeroaccess

On a older very slow laptop with windows XP service pack 3  Norton scan found zeroacces, partially removed it.  I then downloaded the zeroaccess removal tool- fix zeroaccess will not run.  I get error message "tool failure-must 1sy be run without post boot"  this is way over my head but would welcome any help that is offered.  I am currently running a spybot scan but for some reason don't think that will solve my problems.  Thanks in advance for any assistance.