Intel,Altiris Group

I use Endpoint Protection but i can't start Active Scan & Full Scan. It didn't show any error messages.
And I can create new scan (custom scan & Schedule Scan) but it fail to run (show msg "Scan failed") as i have check all endpoint service was started.

What should i do?, my PC is plenty of Virussssss!!! (T_T)

Try to use Norton Security Scan to clean the system first
uninstall SEP using Clean Wipe (Removal Tool)
Then Reinstall SEP

You can download Norton Security Scan from here
http://security.symantec.com/sscv6/WelcomePage.asp

Call support and Open a case to obtain Clean Wipe

CleanWipe will remove all remnants of the following products from a computer:

 · Symantec Endpoint Protection
 · Symantec AntiVirus
 · Symantec Client Security
 · Symantec Sygate Enterprise Protection
 · Symantec Network Access Control
 · Norton AntiVirus
 · Symantec System Center
 · Symantec Endpoint Protection Manager

Note: The following cautions apply to the use of CleanWipe:

- CleanWipe removes all of the listed applications.  You cannot select individual applications to remove. 

- CleanWipe may also remove LiveUpdate.

- CleanWipe may also remove Virus Definitions.

- If you have other Symantec applications on the computer that depend on any of the applications listed above, those applications may not function properly.  You need to reinstall the missing applications after you run CleanWipe.

This might also be your issue

Check the link below
https://www-secure.symantec.com/connect/forums/cannot-run-manual-or-full-scan-client#comment-2545201

Did you try to run scan in safe mode...

As  Acretian mentioned you can reinstall or try to repair SEP...

If the system is already infected there are chances that even the NSS tool might fail to run a scan. In this case, you can map the C drive of the infected system to another good machine that has SEP installed with latest definitions and run a full system scan. After the machine is clean, you can install the SEP client on it. Thanks :-)