Sounds like your PC has been compromised. I suggest investigating the option of getting an AntiVirus program onto the PC without downloading it directly from the internet onto the problem PC. Get on another PC (friend/neighbour?) and use some removable media to download the latest AntiVirus program you're after, and transfer it to the problem PC. You will probably find a slimy virus hiding under the hood somewhere. Thats about the only thing I've ever come across that stops a web browser downloading A/V software. I can't suggest anything else, because operating a PC on the internet without up-to-date AntiVirus software is asking for trouble.