Microsoft has a very good article on this at:
It boils down to the following steps, but includes nice screenshots:
* Set the workgroup name to be the same on all the computers
* Set the network location type (home or work)
* Enable file and printer sharing options
Another important step!
If you have passwords turned on (and I hope you do!)
you will have to create the same user ID on each computer that will share.
if Bob is signed in to the XP PC,
there must be a Bob ID with the same password on the Vista PC.
If Sue is on the Vista PC,
then the XP computer needs a Sue account as well.
The same basic principle is true in Windows 7 as well.
One final note about firewalls.
When you turn on sharing the first time, Windows Vista will ask if you want to enable firewall rules for file and printer sharing.
If you did not answer yes, or if you use Symantec, McAfee or some other firewall,
you will have to set that manually.