You COULD download a file to your USB stick using another computer and then bring the USB stick to your computer for the software to be installed.
However, many virus and spyware programs need to connect to the internet AFTER they have been installed to download the latest list of known viruses and other spyware.
If you dont let it download the latest files from the internet first then it will almost be worthless running any anti virus or spyware software.
If you connect your laptop to the internet just long enough to let the software download the latest virus and spyware details you should not get any more viruses or spyware.
You normally need to visit infected sites to get problems so as long as you dont visit any web sites you should be OK.