If you are in Firefox, press Ctrl+U (view page source) and when it pops up the new window, let us know what it says for source of:
http://somesite.com. I'd be thinking it's some sort of malware/spyware infection that hasn't been detected by your scan program(s). If someone uses some tricks from one spyware, and some from another, it'll often go under the radar since it doesn't meet enough criteria of each to get pulled up.
If you're a windows user, also take a look at your hosts file, in your Windows\System32\Drivers\ETC folder. Typically, it'll have a couple lines commented out at the start, and have a few lines about 127.0.0.1 and localhost. If the size of the file is 500k+, you've got some sort of infection that is redirecting a lot of traffic, most often all the spyware/virus scanners and in doing so, prevents them from downloading/updating.