Can you explain what you mean by SIDs.... perhaps GUIDs (Globally Unique Identifiers)?
A screen shot of what you are seeing may help someone better understand exactly what you're attempting to describe.
If you're absolutely sure that you want to delete them, then......
The first thing you'll likely have to do is "Take Ownership" a link and brief explanation to a handy utility that will help you do this is included in this article here http://www.howtogeek.com/howto/wind...ership-to-explorer-right-click-menu-in-vista/
After that is done you may be able to delete them. If not.....
Right click the item, choose properties, select the security tab and add the "everyone" group and grant that group "full access". You may also need to disable inheritance so as not to have troublesome permissions passed down from the parent container.
When I'm faced with a statement like 'If you're absolutely sure you want to delete them, then....' it makes me wonder whether I'm doing the right thing. Problem is, I don't actually know what they do and what happens if I do delete them. FYI, they are on a second hard drive which I used to use with XP - and it was the only HDD on the machine then. I installed a second HDD and put W7 on it and now use the original HDD for back up. It has some residual folders which I want to get rid of - these SIDs (if I'm using the right terminolog) are some of them, Users is another and Documents and Settings another. Please tell me whether I'm likely to wish I hadn't if I delete them, or whether in the circumstances I describe they have no importance.
In so far as .....
If they are of any consequence now.... I would suspect that if your computer runs fine without that other hard drive attached and you don't seem to be missing any connections to content on it then personally I wouldn't worry about it / them.
I suspect that they are just references to accounts that existed on the former machine and the current machine has no way other than the SID to reference them as being present and existing on the second drive.
Whether or not you should delete them would be determined, I would think, based on their contents and if anything in them is important to you or someone else. That's why I suggested taking ownership and granting yourself access permissions to them so you could examine the contents (if any).
I can't tell you how to handle or what to do with data on your machine. That is your decision to make.
When I'm faced with a statement like 'If you're absolutely sure you want to delete them, then....' it makes me wonder whether I'm doing the right thing. Problem is, I don't actually know what they do and what happens if I do delete them.
When the security identifier is no longer associated with a username, but still exists in the system, you will see this. It can happen when a server is linked up to a domain controller incorrectly, or when only some remnants of a user account has been deleted. You should probably be safe to delete it, so long as you don't plan to somehow re-assign that SID to a user account (this usually doesn't happen or is virtually impossible). But as Trouble says, if its not directly affecting your system, you may want to just leave it. The reason that they are on the second "XP" hard drive is probably because it is still picking up on the old installation and the old user accounts but that is just conjecture. Under certain circumstances, the security identifier will remain, even after the account is destroyed in XP or de-authorized.