I think reason is there are virus already running on your computer, so after inflecting virusus there is no poing installing virus guard but you have to install before inflecting any virusus.
As i see what you can do is try install several other virus gurads and try to remove already infected malwares, else you can plug your hard disk to a another computer (as a seperate drive, not as the boot drive) runnig antivirus sofware, and then scan and remove viruses, this might help but if viruses courpted your system files this won't be able to fix this issue.