The $NtUninstall* folders and associated files in these folders are safe to
remove,
however once deleted you will no longer be able to un-install a patch or
update that was associated with the deleted folder/files.
I would keep the most recent set (last two months just in case) of folders
and delete the older updates.
As a safety net I burned these folders to a CD before deleting them.
$NtServicePackUninstallIDNMitigationAPIs$
$NtServicePackUninstallNLSDownlevelMapping$
Created when you installed IE7.
If you delete them you will no longer be able to uninstall IE7
$NtServicePackUninstall$
Created when you install a service pack.
If you are currently using XP with SP2
then if you remove the folder you can no longer
uninstall SP2.
If you choose not to delete the $NtServicePackUninstall$ folder,
then when and if you install SP3 this folder grow in size
as it will contain both SP2 and SP3 uninstall files.
If you have already installed SP3 then I would leave
this folder as is until you are certain that no bugs have
cropped up after installing SP3.
Note: Once you delete this folder you are stuck with SP3
Warning: One folder you should not delete is: $hf_mig$
(and any folders that are part of/contained in $hf_mig$)
It is a necessary folder for future updates
Also See Doug Knox's page on this issue:
http://www.dougknox.com/xp/utils/xp_hotfix_backup.htm
And:
Is it safe to delete the $NtUninstallKBxxxxxx$ folders:
http://www.pagestart.com/ntuninstall.html
JS
www.pagestart.com