windows messages are popping up and it obviously shows that its a virus , adware or spyware because it shows spelling mistakes and obvious errors and its making me mad it pops up fake windows messages every now and again ( and trust me again is even sooner than now ) wut makes me even more suspicious is that how would windows figure out that i have a trojan r sumthin like that shouldnt it be kind of invisisble to normal computers?
i tryed spybot search and destroy and it destroyed most of the viruses/ spyware and my spyware doctor just expired wen i discouverd i have to scan it and its obviously fake so i need help

one of the messages : Your computer is infected with the last versions of PSW.x-Vir trojan PSW trojan steals your privat ( no spelling mistake ) information, click on this baloon to download the latest version of antivirus