I am not quite clear if your symptom resolves itself or not. Are you saying that you continually reset to factory condition, and after 5 days it suddenly stops working, or slows dramatically. The big question is - does the problem resolve itself after a while (like 2 days may be required)?. This is what Sharam is suggesting - that certain undesirable things start after a time which will (should) eventually complete.
You also seem to be saying certain updates cause this problem, so the first step might be, as an experiment, prevent those updates happening. You can do that by adjusting Windows Update so that it only tells you that updates are available. You can either not download those updates, or download them but not apply them.
At the cost of constant pop-ups trying to remind you, you could leave the PC unpatched and see if the slow-down still happens.
Here are some other suggestions. An OEM version fresh installed from a main supplier like Dell has an immense amount of so-called crud, that you very rarely want. That sort of thing, over time, can become so out-dated that when it starts to try to update itself, what you see can happen. Very usefully, there is a crap-remover which I have found useful here
http://www.bestsoftware4download.com/s-ruiqvrgb-crap-remover-software.html
The other things possible are - prevent drive indexing (component services, services, index service - disable). You may have anti-virus active or Windows defender (as Sharam says), any of which will frequently schedule a 'full scan' at spaced intervals like 5 days. Again these schedule can be simply prevented from running. There are a huge number of things which the naive user is totally unaware schedule automatic checks for updates. Examples most common are Java, Adobe utilities, MS dot-net, quicktime etc etc. You simply have to educate yourself as to where all these utilities have their 'off' setting for automatic updates.
Your symptoms could have a mutlitude of other causes, such as hardware problems, which may account for why your friend sold you what sounds like a reasonably recent and powerful system !! In your case I would seek to download and extensively run every hardware tester I could find. Test the memory, hard drive, video, network. I would also look into checking if there is an updated bios you should install (not too easy, however, unless you have some good knowledge).
As a final resort, if you become convinced you have no hardware faults, and the issue is some incompatibility with Vista, then I would bite the bullet and scrap Vista. Install Win7 instead. Better yet, if the original Vista OEM system allowed a 'downgrade' to XP, then do that (zero cost, or the cost of shipping a DVD from the PC maker). After applying SP3 to XP, you have a system many, many people refuse to upgrade from because it is still so good, and will be maintained by MS until 2014 at least.
Sorry I cannot give you more info specific to Vista, as I personally dodged that bullet !