check this link
http://www.symantec.com/docs/TECH173484
The following fix was added in 12.1 RU1:
SMC.exe fails to start when the policy file (serdef.dat) is corrupt
Fix ID: 2359517
Symptom: SMC.exe will fail to start when the policy file (serdef.dat) is corrupt.
Solution: SMC.exe will now use the backup.dat and server.dat instead of serdef.dat, if serdef.dat cannot be loaded
Manual workaround (if already on 11.0 RU7 MP1 or 12.1 RU1):
In situations where the clients have already been updated to 11.0 RU7 MP1 or 12.1 RU1, this would point to corruption that extends beyond serdef.dat and serdef.dat.bak. Investigate a client and determine if the additional corruption resides in cltdef.dat, cltdef.dat.bak, server.dat, or backup.dat. The only time SMC will fail to start during a policy corruption scenario in 11.0 RU7 MP1 or 12.1 RU1 is if the corruption resides in cltdef.dat and cltdef.dat.bak or serdef.dat, serdef.dat.bak, server.dat, and backup.dat simultaneously. Here are the steps to perform if the corruption resides in cltdef.dat and cltdef.dat.bak:
1. Disable Tamper Protection (if applicable)
2. Stop SMC (smc –stop)
3. Replace the corrupt cltdef.dat and cltdef.dat.bak with known-good copies from another client
4. Start SMC (smc –start)
5. Re-enable Tamper Protection (if applicable)
Note: In 11.0 RU7 MP1 and 12.1 RU1 there is no need to manually replace serdef.dat and serdef.dat.bak.
To manually correct clients that have had their serdef.dat/serdef.dat.bak replaced with one from an incorrect group, make a modification to any policy applied to the group that the client should reside in on the SEPM-side and it will correct the client to reflect the proper group again on the client-side. Note that the policy you modify has to be changed enough to generate a new policy ID number. Verify in the client SEP GUI for the policy number change. When the updated policy is pushed to the clients of that group the *.dat files will be updated with the new policy information.
It is also possible to replace the cltdef.dat and cltdef.dat.bak or serdef.dat, serdef.dat.bak, server.dat, and backup.dat with good files from a working client that is in the group you would like the client to belong to. You can also update the sylink.xml of the client along with the other changes with one exported from the target group. Performing these changes will effectively update the client and it will attach itself to the group you targeted it to.