There is more then one cause of this issue.
This error can result when Norton AntiVirus (NAV) attempts to scan the TEMP file created during the backup process.
You must temporarily disable the AutoProtect feature of NAV to allow the backup process to complete with out error. Please consult your NAV documentation for information on disabling the AutoProtect feature.
The Performance Counters could be disabled or missing. The Performance Counters are a function of Microsoft Windows. ACT! uses these counters to perform various operations from installing ACT! to creating back ups. If you are receiving this error message and it not cause by your AntiVirus you must contact Microsoft to resolve this issue. Please refer to the following Microsoft Knowledge Base document:
How to manually rebuild Performance Counter Library values