Minecraft Server

I was wondering to setup up a server for me and my friends but got some problems.

I downloaded the server.jar file from the homepage and run it via console. It starts up and I can log into it but from the out side it lags like the last day but localy it works.

Think the lag has something todo with the "[Waring] Did the system time change or overload jada jada jada"

Specs:
Win2k
4gb RAM
AMD Sempron 2,4Ghz 3000+
50/10 net speed

Any suggestions to reduce the lag or am I setting it up wrong?

Run it from command line.

Yes. Mean't command line with console.