(Norton) is as good as any other well known anti-virus program, it requires numerous services and running processes that consume system resources and often results in complaints of high CPU usage. Anti-virus software components insert themselves deep into the operating systems core and create files/folders/registry entries in various locations. I have read from other users that Symantec has improved the newer versions while others say differently. However, Symantec products can be difficult to remove and remnants are often left behind which require the use of a special removal tool, otherwise you may encounter problems installing a replacement anti-virus. To be fair, other vendors are also using removal tools for the same reason. Those issues plus the cost factor are the primary reason many folks look for a free alternative.
Norton Internet Security is a suite which means it includes additional features
, in addition to anti-virus protection. Some of those extra features will also consume resources and affect performance.