Your machine is infected. Pop-ups are mostly caused by adware, spyware, and other malicious and unwanted software and using pop-up blocker won't help an infected PC. No single program can detect and eradicate all spyware and I recommend using Ad-aware and Spybot (they are free). Other great free anti-spyware programs would be Super anti-spyware and AVG anti-spyware (formally Ewido).
Running these programs in Safe Mode (tap F8 as the computer boots up and choose to boot to safe mode) can be more effective.
If you do not have an updated anti-virus program always running, you need to get one. Install only one, but you can get AVG, Avast! or AntiVir for free.
I strongly recommend using FireFox, it's a far better and safer alternative to Internet Explorer. I also recommend CCleaner which is a free system optimization and privacy tool.