Install sp2 if you already havent. Make sure all your accounts in your computer are administrators. I would also reconfigure the router. I had this problem once, expect I was using Suse linux, anyway, try that and post back your results.

Good observation; if you can't even ping between machines, you're obviously going to have problems with higher-level sharing functions.

Can all of the computers access the Internet without issue (proving that your basic TCP/IP settings are correct)?

When troubleshooting any network-related issue, the first thing you need to do is to completely disable any firewall software (including XP's built-in ICF/ICS features). Simply choosing the "Disable" option in the firewall program's settings/preferences rarely turns the firewall off entirely; you will need to deselect the preference setting that tells the firewall to automatically start when Windows boots, and then restart the computers. After reboot, verify that the firewall is indeed disabled.
Keep your firewalls dropped until you get things working.

Once you've addressed the firewall issue, see if you can now ping between all machines. If not, post the exact error(s) returned by the pings.