hello,
if you can't even start downloading a security software, i guess the easiest way is to download the install on another computer, put it on a thumbdrive or cd and bring it on your machine. now, if you can't run the install or you can't run the program once it's installed, it's possible that malware you have is capable of blocking security tools. in this case i suggest you to start your computer in a safe mode and run install/installed software in this mode. you may need
safe mode tutorial to do this. the most majority of malicious processes are not active in a safe mode, so you should be able to run a scan. if your not sure what anti spyware is the best to you, try some
free security tools.