D Lirious

Presuming all computers are sharing the internet connection via a
wired/wireless router you are encountering classic Vista networking
insanity.
Realize that Vista was programmed by morons who think you are an idiot:
there is no other way to explain the UAC and the way network sharing is
deliberately obfuscated.
Out of the box Vista expects you will place anything you want to share on
the network in its "public" folders and nowhere else. That the public folder
exists only on your C drive, which there are good reasons not to share,
never seemed to have occurred to the brilliant programmers in Redmond. If
you can live with that arrangement your life will be easier.
It has been my sad experience that it can take multiple tries and reboots to
get the sharing/ownership permissions to work properly. This can be very
frustrating if the computers are not physically near one another and you
have to go back and forth to see if following the instructions has actually
worked (as alas it often does not). The combination of XP and Vista on a
network is doubly frustrating as the rules for sharing network resources are
different (simpler on XP) and Vista has issues, never resolved, sharing with
wireless XP machines.
The problems are solvable but require more effort than it should.
At least you will soon be able to buy Vista SP3, I mean Windows 7, and start
over with new network rules.