I use avg...
not too keen on mcaffee, they are ok I would guess
a lot of apps find questionable things, like cookies...
you can use multiples, but try and only have one run as the active...ie., avg, antivir, nod32...etc.
combo it up with firewall apps, in addition to the ms defender, comodo, zone allarm, etc
which flavor of Vista are you using? the Dell provided version? if so, I think it's hardcoded into the install process and you won't need to enter a key...though if that's the case, don't lose the discs.