I gtta problem with my pc, spyware?

on my taskbar there's a icon of security alert and another is of yellow cross on blue circle and the there occurs a message that your computer is highly infected with virus that infects executable files i don't know wat to install to get rid of this virus .Anyone can tell me that what software should i install or any cd to prevent virus and spyware.AND IT'S SPEED IS TOO SLOW.