Blackworm virus plus internet explorer problems

I have been having a lot of trouble with my computer recently and was wondering if i could get some help.  It started when i received this notice on my screen that is something about the blackworm virus.  It tells me, "There is a security vulnerability from the blackworm virus.  We recommend you DOWNLOAD one of the security software programs to prevent malware infections."  If you click ok, then it will take you to a pop-up ad that tells you to download the program.  This website is not a real thing and i assumed that it would mess up my computer so i closed the window.  I also get a message that says to download a winfixer program.  I know this is a bad thing cause i have had it before, so i closed that window too.  Another problem that i am having is that when i open internet explorer, the window comes up but if a pop-up comes up or if i open a new window, the last window freezes and i cannot operate on it; the new window is fine however.  My computer may be hijacked but i am not sure.  I have had this problem before and so it might be the same thing as last time but if anyone could give me some information on how to fix it, that would be great.  Thanks a lot.

Dan