How can I tell which User has which SID?

A. Perform the following:

1. Start the registry editor
2. Move to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList
3. Select each SID under this in turn and look at the ProfileImagePath and at the end of this string is the name of the user
4. Close the registry editor

If you knew the SID and just wanted to know the user name you could use the REG.EXE command (with Resource Kit Supplement 2), e.g.

reg query "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList\<SID>\ProfileImagePath"
e.g. reg query "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList\S-1-5-21-1843332746-572796286-2118856591-1000\ProfileImagePath"

And again this will show the ProfileImagePath giving you the user.