mirror a folder over the network?

[teststrips]

I currently synchronize a bunch of folders across my network to my home "Server" which is just an XP machine, but acts as a file server. I'm running into problems where the 30 minute sync times don't happen because my client machine falls asleep. I've also had to wait a few times for a document to get to the server before I can get it from another machine.

What I'm looking to do is more of a mirroring, where any changes that I make on my client machine are immediately synchronised up to the server. Any ideas on how to do this?

Edit: freeware or open source software is preferred.