According to this JSI FAQ (http://www.jsiinc.com/SUBO/tip7200/rh7265.htm) "When you open a file on a remote computer, Windows Explorer obtains detailed information about the file, and about the remote share. This process can take a long time over a slow link." That FAQ entry also goes on to recommend installing latest service packs, and includes a set of very specific Registry hacks that may help with speeding up the problem. My guess is that these folders are much, much bigger than any of the others and that WinXP is grabbing more data about them than W98 or W2K try to access—hence the difference in startup/load speed.
Good luck with your situation. See also http://www.tek-tips.com/viewthread.cfm?qid=957524 for some interesting discussions of and insights into the same kinds of problems.
This was first published in April 2005