Why is my computer running sow?

I have a computer and its about 4 years old and its running REALLY slow.And I mean SLOW.I had to go back and correct my sentence multiple times just for this sentence because my computer acts like its freaking old as hell.I mean anything I do runs slow.I have aVG and it says I have like 39 infections that cant be put in the virus vault because its fulls o I tried to remove them and my computer goes supper slow then shuts down.Should I just get a new computer or is there something I can do to make it bearable(this is mostly a gaming computer,we also use social sights like myspace and facebook.But admittedly there have been some indecent sights been looked at on this computer) the AVG full computer scan has to be ran at least once a week.And even then there are little results(BTW its the free AVG version)im gonna try to disc defrag later to see if that helps(this hasn't been done in ages so I hope it works)But if there is something else that doesn't envolve downloading some stupid "virus protector 500" I will not even try it because this computer is in the cr@ps already.And if I do end up buying a new computer im just gonna choose like 3-4 sights I always use and only use those and other trusted sights,I would suggest you do the same right now before your computer does the same as mine(nothing like 40 trojans to make your computer go fatser than the speed of smell,PS don't use Doperoms.com im not sure if it was just a bad file or what but thats what made my computer crash for a week(well its an hyberbole but my computer couldn't run anything without crashing(shuting off randomly) after 2 minutes)