What version of 12.1 are you running? This was a known issue on 12.1 RU2 MP1
Network performance slows on Windows XP with Symantec Endpoint Protection 12.1.2 MP1 installed
Fix ID: 3180923
Symptom: Computers experience network performance problems when the Network Threat Protection (NTP) firewall component of the Symantec Endpoint Protection 12.1.2 MP1 client is installed and enabled. Symptoms include:
- Copying files from a network location over Windows File Sharing takes significantly more time.
- Programs that are started from a network share take significantly longer to load.
- Ping responses take longer to return.
Solution: Modified the firewall to call the correct notification routine for Windows XP.