Trojan viruses, popups, etc.?

I have been getting constant pop-ups advertising things. I would even get pop-ups from websites that I never get including YouTube and...Yahoo. I also get many pop-ups saying "Click here to remove dangerous viruses immediantly!" and it's very obvious that it's just another spyware/virus.

I've also tried a anti-virus called avast! and when I ran it, a trojan virus was detected and came from avast! itself...which was very odd for me.

So what I'm asking is what kind of programs causes these pop-ups and viruses? And where can I get a good, free anti-virus that actually works? Thank you.

Trojan viruses, popups, etc.?house call

Pop-ups are mostly caused by ad ware, spy ware, and other malicious and unwanted software.

http://blog.360.yahoo.com/blog-sAFtLsc8e...

can help you detect and eliminate all spy ware /virus /ad ware /Trojans completely.

Trojan viruses, popups, etc.?matchless

use msconfig to turn off/disable windows messenger.
I use AntiVir which is free from http://freeav.com and scores highly in tests. I also recommend Spyware Terminator from http://spywareterminator.com which can tie in with Clam AV for a thorough scan of your system.

You could also immunize your system by using SpywareBlaster an Spybot to add known spyware sites to the blocked list.
For a good anti virus get AVG. For a good firewall get Norton. These two programs will help protect your computer from viruses and pop-ups.
START - RUN - services.msc - disable and stop Messenger. This worked for me. And of course make sure you have something like AVG free from free.grisoft.com.