From my Understanding McAfee uses McAfee Scanner service and I suspect On Access Scanner service relates to Windows Defender. If you are seeing OAS service from McAfee then please post back the version details of McAfee product Installed (Found on GUI > About link at bottom)
Regarding Site Advisor issue please post back below info to provide with right troubleshooting steps.
1. Version of Site Advisor installed
2. Working on Internet Explorer ?
3. Type of Network connection - whether disabled at startup
4. Whether Site Advisor works fine when On Access Scanner service stops?