First, install LLTD on the XP computer to help Vista/Windows 7 map the XP computer better.
1. Plug router into the modem, then connect all computers to the router.. Make sure they both have an internet connection. Then adjust any router settings such as a wireless security password by using the wired computers only.
2. Create the EXACT same username on all computers, with the same password, and give that user administrative privileges. (Don't use the default "adminstrator" login.. Create a new admin user.)
3. On all computers, using the new username and login, change the "Workgroup" to be exactly the same.. (RIGHT click on "My Computer", choose "Properties", then click on the "Computer Name" tab. When that opens, click on the "Change" button next to "To rename this computer or join a domain". On the next window, place a dot/tick in the "Workgroup" selection and give the the workgroup a name..) Make sure all computers have the same workgroup name.
4. Make sure all computers have "File and Printer Sharing for Microsoft Networks" enabled. Open the Control Panel/Network Connections icon, then RIGHT click on the "Local Area Connection" icon, choose "Properties". When that window loads, make sure the box next to "File and Printer Sharing" has a CHECK mark in it.
5. If you're wanting to share folders between computers, On computer 1, share out the files, folders and printers by opening Windows Explorer, RIGHT click on the C: drive, choose "Sharing and Security", then click on the line that says: "If you understand the risk but still want to share the root...". At the next window, in the "Network Sharing and Security" section, place a CHECK mark in the "Share this folder on the network", then follow the prompts to share the drive. Click on Apply, then OK.. Perform the same thing on computer 2, etc... You should now be able to see and share folders between computers.
Please note, sharing the entire C: drive per the instructions above is not safe.. IF you're going to share files between computers, it's best, after following the above instructions, to then share out a single folder, such as the "My Documents" folder.. Once the "My Documents" folder is correctly shared, you can then return to the C: drive and "unshare" it.. After doing so, the "My Documents" folder will remain shared but the entire C: drive will be invisible.
Sixth, if connecting to a shared XP printer from a Vista machine, instead of installing a new "network" printer from the Vista computer, install the printer as if it was a "local" computer using the instuctions below. (Yes, I know it's "network" printer but please try the instructions below.):
Go to Control Panel. Choose printer. Then choose Add Printer.
Choose Add a local printer. Click on Create a new port. The default in the drop down box is Local Port. Do not change that. Click Next.
A dialogue box will appear asking for you to enter a port name. Type in the \\computer name\printer name of the XP machine....ie. My computer's name is basement and the printer name is EpsonSty so I typed in \\basement\epsonsty
Next, see the link below for other networking tips:
Hope this helps.