Lsass.exe attack(?) on Port 500

Since about 1 hour somebody is trying to access my PC in a rythm of ten seconds from two different IP's. Never had this. Some ideas? I blocked it, but its getting anoying.

2006-10-12 14:01:58
reject TCP/IP
receive datagram
C:\WINDOWS\system32\lsass.exe
0.0.0.0
200.161.208.25
500 500
PID: 844; Connection: 37

and

2006-10-12 14:02:10
reject TCP/IP
receive datagram
C:\WINDOWS\system32\lsass.exe
0.0.0.0
201.17.6.14
500 500
PID: 844
Connection: 37

It is not the isass.exe which is known as a worm.

Both adresse are listed in the Spam Database.
http://www.dnsstuff.com/tools/ip4r.ch?ip=200.161.208.25
and also here:
http://www.us.sorbs.net/cgi-bin/lookup?NAME=200.161.208.25

Mein Gott das nervt