Ok, so assuming you're using XP
Pro - which you
must be or this
won't work...
Prepare the PC:
Start > Settings > Control Panel > Windows Firewall. Go to "Exceptions" and make sure "Remote Assistance" is ticked. If it's not even there then you need to add it. Click "Add Program..." and browse to "C:\WINDOWS\system32\sessmgr.exe", OK it, and make sure it's ticked.
Get your PC's IP address by going Start > Run > "cmd" {return} > "ipconfig" {return}. Make a note of it.
Switch on Remote Desktop by right clicking on My Computer, the going to the Remote tab, and ticking the box for "Allow users to connect remotely to this computer".
Open up your browser and go to
http://www.whatismyip.com, and take note of the IP it gives you.
Configure your router:
For Linksys I think the WebCP address is
http://192.168.1.1, so go there. Find where you can configure port forwarding, and setup TCP port 3389 to forward to the IP address of your PC. I can't explain this in detail, since I don't know the details of how Linksys do their port forwarding.
Connect to your PC from your Mac:
Download and install the Remote Desktop Connection Client for Mac, already mentioned in this thread. Enter the IP address that you got from whatismyip.com earlier. You
should be good to go.