Unknown account removal

In a 'permissions' dialog an attempt to remove an UNKNOWN ACCOUNT prompts that inheritance first must be disabled. Two options are offered...

1) Convert inherited permissions to explicit permissions on this object
2) Remove all inherited permissions from the object

Which should be chosen, and will other accounts be affected? Or, would it be prudent to ignore the unknown account?

This is an "Sxxx" account and does not appear in the USERS account list, nor with other normal registry accounts.

Inherited permissions come from the folder above the current folder, or one even further up the folder tree. the best way to deal with this is to go up a folder level and check if the unknown user S.... is there. When you get to a level that does not show the unknown user go back down one level and you should be able to remove the user there. it may also be that the user will actually be present at the root of the drive and so need removing there. You only need to delete the user once and allow the change to cascade down to all lower levels. Occasionally you may get an error part way down the cascade, due to a past change in permissions, if so then start another deletion cascade at this point.

These type of unknown users are usually due to the NTFS file system being accessed/used by More than one copy of windows due to re-installs and so deleting the user should not cause any issues.

If however, you have a still operative multi boot system that accesses the data from two windows installs then you should not delete the user as it is likely to be a user from the inactive system