You're not trying to use VPN to connect to a remote mac, are you?
I'm pretty sure that doesn't work unless there's a VPN server using the same transport (L2TP in this case) on the Mac. None of the native services act as vpn, as of Tiger.
The advice given up to this point was based on the assumption that you were on a local Lan using either Ethernet or wifi. If we were mistaken, please accept my apology.
If you actually are using ethernet or wifi and not VPN, and I confused you with my reference to IPSec, I also apologize.
The last thing I know to check is to make sure you actually have a real shared resource (not an administrative-only share) on the windows box. You can't access any admin-only resource whose sharename ends in $ remotely on a Mac.
If you need access to such a share, you'll have to assign it an additional name under the Sharing tab of the resource's properties pane. (Right-click, properties; alt-enter while highlighted; alt-doubleclick) Windows will get annoyed when you do this, but if you click yes/ok, it'll do it.
Note: Sharing an entire volume presents a SUBSTANTIAL SECURITY RISK on any public-facing PC. Exercise caution when doing this and make sure your firewall is blocking incoming SMBs from the internet unless you're absolutely sure you need to allow this access over the internet.