theres a saying, if it aint broke dont fix it. normally viruses are detectable in some small way shape or form, taskmanager and looking at processes most cases works wonders for self detection.
if your a computer geek like moi, then you'll know what should be on the processes on taskman and whats not supposed to be there. if you get to it before it really digs in then youve got a chance to kill it manually as most viruses knock out and disable other .exe's including any AV you have installed.
a regular scan with your AV software would pick up something anyway if it was already on your PC due to whats called a 'drive by' where you only visit a site and it auto downloads into your TEMP folders, if its on a timer then it will install itself say two days later or something.....i know as ive had it happen :lol: