Chances are that your system is infected with spyware/malware that won't let
you download spyware/malware removal programs like Spybot, Ad Aware, MS
Defender, etc. Some crap is just really nasty like this.
Lets see if we can get some information to disect...
Get Process Explorer:
http://www.sysinternals.com/Utilities/ProcessExplorer.html
Download it, unzip it, run it, then do a File -> Save As... saving the file
somewhere where you can get to it.
Open the text file and post it's contents back here. This way we can see if
something is running that isn't supossed to be.
-Dan