It won't ruin it.  It will slow it down if you have all the different programs running all the time.  The only program that you need to run all the time is the antivirus.  I would not install it personally because I have no use for Norton Products.  That is my opinion only, others will disagree.

fubugirl, as it is commonly known on the Internet as "Norton System (doesn't) Works" then I would save your money.

Anything Norton (Symantec) will slow down your system.

Look for AVG anti virus.

Read ChrisRLG's awfully coloured Web site but good info: http://www.cjwd.demon.co.uk/compsafetyonline.html