Computer Infected with Virus

My computer was infected with virus recently because it keeps booting up on its own very frequently. So it was sent to the technician for about more than 1 week before it was ok and that bugger installed a trial copy of Anti Virus XP 2008. This is a spam software because it lead me to a site to pay for the full version costing RM150 which I decided to use my credit to pay. Upon payment, I realized that I was charged RM350 instead of RM150 as stated by the so call anti virus web site. After that I got the password to key in for full execution of the software which maliciously installed anti spam guard which keeps prompting up for me to pay further for the full installation. This is spamming my computer with unnecessary messages and I just cannot removed it from my PC. Moral of the story is never trust your technician to know everything and don't pay to install anti virus software.