Free anti virus software packages are AVG, Avast, AntiVIR. I use AVG and have done for years. For free spyware protection use Ad Aware SE personal, Spybot search & destroy. For free firewall use zone Alarm or Sygate or Windows SP2 built in firewall. Those are the basics you should have and keep up to date. A good place to download the latest versions is
www.majorgeeks.com.
MS internet explorer is over 90% of the browser market and therefore it is the primary target for browser hijacks. Change your browser to Firefox for better internet security. It's a great browser anyway and with the plugin Ad-Block you can block elements of the page you don't like ie the advertising banners. Also there is a plugin that will give a risk rating to every site and a popularity ranking. If you click a link and it looks like your bank website but the risk is high and the ranking is low and the site is hosted in russia then that is not your bank.
Hijackthis is a great program too which you can run to create a log file. You can post the log file on the forums over at
www.spywarewarrior.com and work through any spyware virus and hijack problems with expert help. Read the forum rules first before you post.