W32.spybot.worm

hi all;
i am facing a w32.spybot.worm in windows/system32 folder and file name is phr.exe is effected that is shown by norton anti virus when this virus appear the machine is out from LAN and when i restart the machine its work proper for 2 or 3 hours. i am using windows XP Proffesional with service pack2,symantec antivirus corporate edition Installed antiviruses software are updated. i has been run anti virus software on safemode but not detect any virus from these anti virus software. please advise me how to remove this virus, your cooperation would be highly appreciated.
Thank You