you might be having a conflict with a firewall or some other application that changed the NDIS.SYS file.

Try expanding ndis.sy_ in your I386 directory to %windir%\system32/drivers/

If you are using IPX/SPX/NetBIOS Compatible Transport protocol you may simply have a NIC configuration problem which this link will help with (if you do not have the IPX/SPX protocol installed this does not apply!) -
http://support.microsoft.com/kb/818326

If you are using the online installation download the full Service Pack 3 from this link -
Windows XP Service Pack 3 Network Installation Package for IT Professionals and Developers
http://www.microsoft.com/DownLoads/details.aspx?FamilyID=5b33b5a8-5e76-401f-be08-1e1555d4f3d4&displaylang=en
Save it to your hard drive and after the download completes double click the file and install but first make sure your browser, email client and anything else that connects to the Internet is shut down.

PS - I would also burn that Service Pact to a CD as sometimes it is handy to have for a quick re-installation of the SP if installing a wayward application attempts to replace newer DLLs with older ones.