This article doesn't solve the mapping problem, but it lets you open a Sharepoint folder in Windows Explorer on Win XP...

Map sharepoint document folder to a network drive - Outlook by the sound: "'\\Servername:portnumber\sites\mysite\Shared%20Documents\'
The port is not supported by Windows folder sharing. The UNC name folder sharing path can only use the machine name appending the folder.
The port concept can't be applied here; it should be used with the HTTP url.

We can specify http url as the sharing path. The steps is listed for you below:
1. At the File explorer, please click the 'Tools->Map Network Drive'
2. at the window, please click the link 'Sign up for ...'
3. at the pop-up window, click next, next and input the url. For example: http://mysps:2005/Shared20Documents
4. click ok to set up this mapping.