Without up to date antivirus software your PC will become infected, virus removal is complicated and time consuming!
Download and install AVG Anti-Virus FREE Edition from Grisoft:
Click this link: http://download.cnet.com/AVG-Anti-Virus-Free-Edition-2012/3000-2239_4-10320142.html
Then click on this button:
This software is free for individual domestic PCs, see their licence conditions on their web pages. It is VERY important to keep this software updated by scheduling automatic updates.
WARNING: if you do not wish to pay for the software, be careful to download the FREE version of the software not a TRIAL version