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Ok. First take a deep breath and gather your patience. I was in exactly the same situation when I bought my new HP laptop. I wanted to network it to existing two WinXP machines. It wasn't quick but it did work, eventually. Try some of these steps:

1) Firewall settings - one of the biggest hassles. To get started I suggest you FIRST disconnect your network from the internet (for your protection), then disable ALL firewalls (including Windows firewall), and work to get your home network operating internally. What I learned on my home network is below. See the below link to a Microsoft article on file and printer sharing.
1.1) My WinXP machines have ZoneAlarm and I needed to ensure that all machines on the network were in the trusted zone. See Zone tab of Firewall settings in ZoneAlarm free version.
1.2) My Vista laptop came loaded with Norton Protection Center including Norton Internet Security. Go to Norton Internet Security tab, open settings, scroll to bottom, open internet security and firewall options, open advanced settings, open configure, change default inbound NetBIOS, inbound NetBIOS name, Block Windows File Sharing to ALLOW. Configuring this beast of an app required a lot of trial, error and googling. There are some hints in the MS article on file and printer sharing at http://www.microsoft.com/technet/network/evaluate/vista_fp.mspx

2) Ensure sharing is enabled on all machines. This is fairly straightforward and is well documented on the MS site and elsewhere.
Link for Vista: http://www.microsoft.com/technet/network/evaluate/vista_fp.mspx
Link for WinXP: http://www.microsoft.com/downloads/details.aspx?familyid=FD7FD48D-6B4A-448E-A632-076F98A351A2&displaylang=en

3) Workgroup name. Ensure you are using the same workgroup name on all machines. WinXP and Vista machines have different default workgroup names.
WinXP: http://www.microsoft.com/resources/documentation/windows/xp/all/proddocs/en-us/windows_network_changename.mspx?mfr=true
Vista: http://windowshelp.microsoft.com/Windows/en-US/Help/5723d061-b884-40da-b5e7-94a55f6fcead1033.mspx and http://windowshelp.microsoft.com/Windows/en-US/Help/baab4f1a-2461-482d-bb2e-c996a197e35f1033.mspx

4) Download and install Link Layer Topology Discovery (LLTD) onto the WinXP machines. Just a nicety, this enables the network map in Vista to show all machines on the network. http://www.microsoft.com/downloads/details.aspx?FamilyID=4f01a31d-ee46-481e-ba11-37f485fa34ea&DisplayLang=en

5) And the final step: check the registry on your WinXP machine per MS article 913628 that I found by searching MS for "error message when you try to access a Windows XP-based network computer: "You might not have permission to use this network resource". See the article at: http://support.microsoft.com/kb/913628. This finally did the trick for my network.

Keep pounding away at it. It will eventually be worth it. It took me almost a week to get everything working right.

Best of luck to you …

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