Hi Squid13,
As you have your computers in a work-group then authentication details need to be the same.
1) Both computers need to be in the same workgroup e.g. "MSHome" details for XP and Vista are more or less the same.
Join or create a workgroup - Windows Vista Help
2) Click on Start > Control Panel > User Accounts.
If you do not already have a password on your XP machine you will need to set one.
Then repeat the steps on the Vista machine so both username and passwords match this should resolve your authentication issues.
3) If this does not resolve your issues you can change some share permissions on the Vista machine. (I assume you are trying to access a network share?)
>> Right Click on the share, left click on the "Sharing" tab along the top. Then left click on "Advanced sharing" tick the check box "share this folder" then click on "permissions" under everyone check "full control" press OK and then OK again.
Finally click "Share" then in the users box type "everyone" and allow full control or just make the account you are logged on as owner.
Note full details on enabling and using advanced sharing here:
Advanced Vista File Sharing
If you still cannot authenticate then your firewall may be blocking something.
in the "run" or "start search box" of the start menu type "firewall.cpl" temporarily select disable and see if this resolves your issue if it does we can provide more info.
Cheers,
Benmarshall5364