For about £20 you can get Nod32, a virus scanner that runs in the background and uses nearly no resources. It isnt power hungryl ike Norton and is known as a very good Anti-Virus software.
See
http://www.eset.com/ to dowmnload.
Then you need a firewall, Most of the free ones such as ZoneAlarm will do the job adequately.
Free to download from
www.zonelabs.com
Then, once you have your PC, forget about usin gInternet Explorer and get yourself Firefox for a better all round browsing experience.
You should also download Lavasoft Adaware and run it every week or so to clear out any adware anf spyware junk.
www.lavasoft.com
Then download CCleaner and run it every couple of days to keep your PC clean from unneccesary temp files, cookies, temp net files, etc.
It can also be used to keep your registry nice and clean.