This could be due to a known issue with webclient, fixed in XP Service Pack
1.
Assuming you don't use "Web Distributed Authoring and Versioning (WebDAV)",
here's a possible workaround:
Type the following command to determine whether webclient is running:
sc queryex webclient
If the STATE shows RUNNING, then run the following command to disable it:
sc config webclient start= disabled
The initial slowness in accessing file shares should not occur on the next
boot. To avoid a
reboot, you could run the following command:
sc stop webclient
Hope this helps,
Arvind