I'd advise nukeing a PC as soon as you buy it, i.e. format the HD and reinstall the OS, they come full of 'bloatware' (crap you'll never use or want) these days, which all slows things down, particularly at boot up time...this will also reduce the RAM requirements.
It also removes the usual pointless dual partition that vendors are so keen on.
And as long as you don't use IE (firefox or chrome are much better IMHO) to go on the net then you don't need any anti virus, unless you''re going to some odd pages, or do loads of downloads of specialist stuff, so that can speed things up too.
And like the man said earlier, defrag your HDs on a regular basis, and don't go over 70% full on your system disk.
I wouldn't trust a program like this, how could they have possibly tested it enough?