You must be running RU5 or higher for Windows 7.
This KB may offer a solution.
http://www.symantec.com/business/support/index?page=content&id=TECH97223&locale=en_US
If this fails to resolve your issue, can you post the SEP_INST.log?
From start - run - %temp%\SEP_INST.log, search for Return Value 3 and post the 10 lines above the return value 3.
Thanks,
Thomas