Just use classical \"networking\".
Share the directories / drives on both computers (right mouse click on device / directory and select share)
Enable sharing on Both VISTA and W7 machines
should just work. Homegroups to me seem an unnecessary complication and only (currently) work between W7 machines.
BTW if you have a Printer connected to one of the machines you can share it simply by the following
enable the share on the computer the printer is on and call it sharedprinter
now on the OTHER machine (client machine) do the following
ADD LOCAL (NOT NETWORK) Printer
Select Port type LOCAL
Name of port is \\computername\sharedprinter
now from your dropdown on your client machine just select printer / model and load the driver. If you can't find it Windows update should give you most of the common drivers.
Incidentally this is the way to share a printer say between computers 1 running a 32 bit OS and the other running a 64 bit OS. Works both ways round.
If you've got one of these \"new fangled\" wireless network printers then do the following
still add LOCAL printer (NOT NETWORK PRINTER)
now create TCPIP Port
name of port is \\printer
ip address where the ipaddress on your lan is something like 192.168.2.8
What you are doing is to ensure the printer driver is the correct one for your LOCAL machine. As before both 32 and 64 bit systems can connect to the printer this way.