Thanks for hints!
In case of anyone having similar problem - it can be easily solved.
Windows 2003 Server deserves painful death! It just stopped answering
pings and most of the network traffic was terribly slow (MS SQL Server
worked, though... strange). Restarting the Windows server helped -
everything works now. We could not do it before (very important users,
MS SQL Server, other wonderful things). I suppose the problem was not
in any way connected with PostgreSQL. Someone told me that many W2003S
configurations (especially with some server services running) require
weekly restart to work well. Just in case, PostgreSQL server will be
probably moved in some time to Solaris or some *nix...
Thanks,
Bart Golda