Nothing updates

My first thought is that it could be an undetected virus or spyware causing this. Not sure what else there could be, but I'm sure someone else will come up with something. you could try running a bootable live antivirus cd like Dr. Web or BitDefender.

Sounds pretty dubious to me, as bpdude said, it does sound like an infection is causing this issue.
If you can download and run Malwarebytes it may find something (you may not be able to update it, but the download should be pretty up to date anyway. Unfortunately they don't have a portable version, so it has to be installed and run on the local machine due to requiring certain drivers)
Have you tried updating via a different browser? (which one are you using?)

If none of these suggestions work, I suggest you start a new thread in the 'Virus/Trojan/Spyware Help' section and give then all the details they require to help you.