See the link to MS KB article 290403.
If you are accustomed to the Win2000 style folder security tabs via Explorer right clicks, check out the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa forceguest value.
I have been using the "simple" XP Pro sharing and security tab at home for ages. This provides easy access to file shares between my home desktop and work laptop.
Recently I discovered I needed to provide my daughter with modify rights to a Program Files sub folder. Sharing it, and then tailoring network access to it seems crazy, but thats what the "simple" interface allows.
On XP Pro, to access the "classic" security options, change HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\forceguest from 1 to 0.