I use AVG 7 anti-virus software by Grisoft, which is available free for personal use. If you have a broadband connection, I advise you to use the auto update function to update your Anti Virus every day. click here
Some people and companies want to see what you do on the internet, and monitor your habits, and will deposit software on your PC without your permission to do this. Install SpywareBlaster software which is free to protect your computer, and install SpyBot Search and Destroy and Adaware to scan for spyware and remove it. All programs have the facility to check for updates to their databases and should be updated regularly.
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Also delete all your temporary internet files - internet explorer - tools options
this should help