There's a few ways to do this;
1.) DFS folder replication will work, there's a tutorial here and here's the MS webpage.
2.) Robocopy in mirroring mode will work as well, but it won't key off the fact that a file has changed. Here's a script that I shamelessly copied from somewhere, (why re-invent the wheel, eh? :) that I used when migrating large amounts of data from one share to another. (A big bonus with this is that it will do incremental mirroring, i.e. run it once to replicate, then run it again and it will only copy over the files that have changed.
SET _what=/COPYALL /B /SEC /MIR
/COPYALL :: COPY ALL file info
/B :: copy files in Backup mode.
:: /SEC :: copy files with
:: /MIR :: MIRror a
SET _options=/R:0 /W:0
/LOG:worstation_copy.txt /NFL /NDL
:: /R:n :: number of Retries
/W:n :: Wait time between retries
:: /LOG :: Output log file
:: No file logging
:: /NDL :: No
ROBOCOPY %_source% %_dest% %_what%
3.) If you use HP in a large corporate setting and have a vendor agreement or something set up with them, check into HP StorageWorks Storage Mirroring, however it's a bit heavy-handed for two windows folders.
4.) Finally, there's also, (beta warning! Beta warning!) Windows Live Mesh, but it's.. You know, beta. :)
Personally, I'd go with the robocopy option.