A New problem I face with mcafee Antivirus 8.5.0.i

The client Pc's are installed with windows XP
The Antivirus we use is Mcafee 8.5.0.i with Antispyware 8.5i
we scanned and removed some viruses like trojan recently using Mcafee and now the system needs latest updates of antivirus and updates are done.
The problem is

1. In some systems, an automatic message is being popped up often as "Virus Scan Alert!" with following informations
Name: C:\windows\System32\svchost.exe:KERNEL32.LoadLibraryA
Detected AS: BO:Writable BO:Stack
State:Blocked by Buffer Overflow Protection
I scanned all the drives again for virus and the result is nothing found.
I also tried to delete the file "svchost.exe"but i am unable to do it.
I hope it is not a virus but i have to fix it.
I even uninstalled the Mcafee Antivirus software and reinstalled newly in that system and again it(the popup message) is coming.
After new installation of Antivirus, when i try to update the antivirus with latest update, I also find this error.
A message box with heading "SuperDAT Notify" and with message "Error:Unable to backup existing file. Not Enough rights: Line:18"
and "buffer overflow" message is also coming often.

Problem 2:
In some systems, when i scan for virus it shows the message as virus detected but the virus file is not visible. I cant find the file to delete it.
I need solutions for these problems.
Pls give me a good result.