The most recent service pack for Win2K is SP4, not SP2. It should most definitely be installed, along with any other critical updates available on
http://windowsupdate.microsoft.com if you have any concern for staying spyware/virus/hacker free.
As for Windows XP, right after installing it on a new system is a great time for installing SP2, as it is then less likely that some other program or malicious software will interfere with its installation. At any rate, there is always a vocal minority to report system problems after installing the latest Windows service pack (which, to some degree, is unavoidable due to the 100's of millions of different software/hardware configurations out there), but overal fewer users had serious problems with WinXP SP2 deployment than e.g. WinXP SP1.
I believe Microsoft is dropping support for Windows XP without SP2, which makes upgrading more important as some future WinXP updates may only work with SP2.