If you are connected to the internet you should have a have a firewall to help keep hackers out of your computer. I would recommend Zone Alarm, it is free and is recommened by many. Check it out at
http://www.pcworld.com/downloads/file_description/0,fid,7228,00.aspEveryone hates the spyware that can steal your personal information or track your every move. Be sure to check out Spybot Search and Destroy
http://www.pcworld.com/downloads/file_description/0,fid,22262,00.asp. It should get rid of alot of spyware and it may decrease the number of ads that just pop up on your computer.
While firewalls and anti spyware programs are nice, dont forget about the original threat: viruses. If you want something free check out AVG Anti-Virus. It seems to work very well, especially considering the price.
http://www.pcworld.com/downloads/file_description/0,fid,15202,00.aspTired of all those popups? Give Pop-Up Stopper a try.
http://www.pcworld.com/downloads/file_description/0,fid,8060,00.aspThere are alot of other useful utilites at
http://www.pcworld.com/downloads/collection/0,collid,1247,00.asp