virus attack

now i am really having shit with this computer.
everything is in deutsche and there is apparently some problems with the antivirus pro software. it keeps on saying the computer is not protected and the antivirus is not on. but when i wanna siwtch it on, something else comes up....asking me to register or buy or something. i think the computer only has a demo version

i cannot open new pages and pop-ups will automatically disappear.
it has not been updated for ages and i think i accidentally click the update page and gosh, the virus list....trojan...not one but a few. die lah.

shall i download another free antivirus software?? avira? i wonder if it will work. it is legal right?