The simplest way is to share them just like you would share a folder on your computer.
To turn on File Sharing in Windows, right click on the drive in My Computer and choose Sharing and Security... and then choose to enable it.
To turn on File Sharing in Mac OS X, go to System Preferences and click on the Sharing Preference Pane. Then check off the check box beside "File Sharing". Lastly, click the Options... button and check off "Share files and folders using SMB".
To connect to the Mac from the PC, if set correctly the Mac will show up in the Network section. You then just need to give it your username and password you use to log into the Mac. If it does not show up automatically you can manually enter \IP_ADDRESS_OF_MAC\ in the Address Bar in Explorer and then enter the Username and Password - on some setups you may need to prefix the username with "\IP_ADDRESS_MAC\"
To connect to the PC from the Mac, by default in 10.5 the PC computer name will show up in the Sidebar in the Finder under the Shared section. You can click on the PC and then press the Connect... button. If it fails to show up, in the Finder from the Go menu choose "Connect to Server..." (Command-K) then simply enter
smb://IP_ADDRESS_OF_PC and then enter the username and password that you use to log on to your PC.
Permissions for various accounts can be set in the Sharing and Security window on the PC, and in the Sharing Preference Pane on the Mac.
Since you're using file sharing - obviously you'll need to make sure the other computer is on. You will also want to check out another couple of questions here on ServerFault: 4599 and 5564