This issue has been answered many times in these forums. Do a search on backup and also on backup2ws for solutions/workaround.
In short the issue is to do with limitations of smb and tar, and also the inability of browsers to restore from files bigger than 2Gb.
The problem is unrelated to Win2K or XP ability to handle large ie >4Gb files sizes.
The answer is that you can only use the backup to desktop feature if your resultant backup file is less than 2Gb in size.
The alternative is to instal and use backup2ws (available from dmay contrib site) and set the file size limit to 2Gb (or just under). Backup2ws will split the backup file into a number of 2Gb pieces which can be successfully copied to a workstation and restored from.
Hope this helps
Regs
Ray