WinXP login prompt takes more than 1 minute to appear (only if connecting to Win2k)

I've several laptops (IBM ThinkPads) within the same IP network (all
connected to one switch). Some laptops are set up with Windows XP SP2
and the others with Windows 2000 SP4.

Connecting from a Windows 2000 laptop to another Windows 2000 laptop:
Start - Run... - \\win2k1\c$ - Ok
-> I get the login prompt within a couple of seconds.

Connecting from a Windows 2000 laptop to a Windows XP laptop:
Start - Run... - \\winxp1\c$ - Ok
-> I get the login prompt within a couple of seconds.

Connecting from a Windows XP laptop to another Windows XP laptop:
Start - Run... - \\winxp1\c$ - Ok
-> I get the login prompt within a couple of seconds.

Connecting from a Windows XP laptop to a Windows 2000 laptop:
Start - Run... - \\win2k1\c$ - Ok
-> I have to wait for more than 1 minute to get the login prompt!!!
However, if I do it from the Command Prompt with
net use z: \\win2k1\c$ /user:testuser testpw
it connects within a couple of seconds.

Any help and explanations are very appreciated.

Regards
Stefan