With McAfee 8.7i the on access scanner fails to start, and while attemting to start the service manually the process faills with the following error,

Event Type: Error
Event Source: McLogEvent
Event Category: None
Event ID: 5004
Date: 10/02/2009
Time: 15:38:54
User: NT AUTHORITY\SYSTEM
Computer: WARDEN
Description:
Could not contact Filter Driver.
Error = 0x7d1 : The specified driver is invalid.

1. Click Start > run > Regedit {enter}
2. Navigate to HKLM\System\CurrentControlSet\Services
3. Locate the mfebopk sub key
4. In the right hand window locate the ImagePath key
5. Change the entry from “system32\drivers\mfebopk.sys” to C:\Windows\System32\Drivers\mfebopk.sys”
6. Repeat the procedure with both of the following Keys

HKLM\System\CurrentControlSet\Services\mfeapfk
HKLM\System\CurrentControlSet\Services\mfeavfk

For each one replace the path to the file with the full path (Usually acdding c:\windows to the beginning)