1st.you should have anti-virus if u r using internet then norton will be good.
2nd.if you keep some important document in your computer then try to make different accounts 4 it.
3rd.Make sure that you use all the latest version of application like internet explorer version 9.
4th.Try to Download all the things from trusted sources only.
5th.Having all the security can also create problem,so you must have back up of all your file.