Tip 1: More Basics
As long as you're checking the basics, open the System Control Panel and go to the Computer Name tab. You can name each computer as you like (as long as each one has a different name). But make sure the Workgroup name on all systems is the same.
Given the limitations of previous versions of Windows and my own paranoid tendencies (at least when it comes to Windows networking...), I only use numbers and letters for both names and always use ALL CAPS for the Workgroup name.
Figure 2: Computer and Workgroup Names
I'm assuming that most LANs will be running off a single router with clients set to be DHCP clients (obtain address automatically). This should put all clients in the same Class C subnet with all IP addresses having the same first three "octets" (Example: 192.168.0.X).
TIP: Each of the numbers between the periods in an IP address is called an octet. Each octet is an 8 bit number.
But, what the heck, check the IP addresses on each computer anyway, to make sure they're following this rule. XP makes it easy, as shown in Figure 3.
Figure 3: Verifying IP address info
The last basic we'll check is the Server service that actually shares the files. This is normally started when Windows boots up, but sometimes those helpful little "Wizards" that come with various pieces of networking gear or applications can get carried away and disable the service.
Open the Services Management Console (Start > Programs > Administrative Tools > Services) and scroll down until you find the Server service shown in Figure 4.
Figure 4: The Server service
Double click it to open its Properties window and make sure it looks like Figure 5.
Figure 5: The Server service's Properties
If you make any changes, close each window by hitting the OK button, and you'll probably want to reboot. If no changes are made, just close [X] or cancel all windows. [Thanks to Dan Withers for submitting this Tip!]