After enabling "Use FIPS compliant algorithms for encryption, hashing, and signing" (FIPS mode), Windows XP and Windows 2003 clients appear offline in the Symantec Endpoint Protection Manager
This is a WinINet issue. The included version of WinINet on Windows 2003 is 6.0.3790.3959, which seems to have a problem with TLS 1.0.
The solution is to upgrade Internet Explorer to version 8.0 or greater.
Internet Explorer 6.0.3790.3959