It sounds like it may be firewall issue since you are using a mixed mode network - wired/wireless and this can sometimes appear as 2 subnets.
Open OneCare, click on change settings, firewall tab.
First, make sure that the network is identified as "Home or Work" on both PCs. Next, go to advanced configuration and the Ports and Protocols tab for the firewall. There are two entries for File and Printer sharing. The local subnet option is checked by default if the network is "Home or work." Add a check mark to the other entry - Internet and subnet.
If this does not resolve it, please contact OneCare support -
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve