This is far from the only method, but it's likely the simplest.
Connect the two computers directly via firewire. Avoid using a firewire hub, and don't have firewire hard drives and the like on the same bus as the two computers.
On the Mac:
1. Go to System Preferences -> Network. Create a new service using Firewire as the interface.
2. Configure your FireWire interface manually, giving it an IP of 192.168.2.1 with a subnet mask of 255.255.255.0 (leave other fields blank).
3. Go to System Preferences -> Sharing.
4. Enable File Sharing to Windows clients ("using SMB" on 10.5). Be sure to active your account in the Options window.
5. Also enable FTP Sharing (sometimes this works better than SMB).
On the PC (assuming XP, probably similar in Vista):
1. Right-click on My Network Places -> Properties. Your FireWire connection should be there.
2. Change the IP address of the FireWire connection to 192.168.2.2 with a subnet of 255.255.255.0.
3. To use SMB: in a browser window, type \\192.168.2.1\yourname where yourname is the short username you use on your Mac.
4. To use FTP: in a browser window, type
ftp://yourname@192.168.2.1/yourname
Best of luck!