Opening ports - UDP 500 & 4500 ?

I am hoping someone can help me with this problem.. I'm running Windows XP with SP2, and I believe it is IPSec related.
I have a VPN client I use to connect my laptop at home to my work network. It used to work, and then all of a sudden quit. I ended up doing a Ethereal trace while trying to connect and I see a packet complaining that I am not listening on UDP port 500.
I looked at netstat and there is no one listening on port 500. I do have lsass.exe running in task manager though.. which I think is in charge of starting that listener.
I checked on my work computer and in deed I was listening on ports 500 and 4500, and they had been started by lsass.exe.
I do not know what the difference is between my home and work computers, to make one listen on those ports and one not.

Is there a way I can start listeners on ports 500 & 4500??

Thanks in advance!