Everybody makes mistakes, oh yes we do. Your mom and your dad, your sister and brother too.... Wait.. No. Nevermind. If you think that you've removed something with HijackThis that you shouldn't have, don't sweat it too much. HijackThis, by default (unless you're running it off of a CD or some other read-only media) makes a backup of everything that you remove. So, in order to get something back you just go to a different menu in the program.
Open up HijackThis, and click 'Do a system scan only' (I'll take you through it this way, because previous versions of HJT didn't have that first page). Next, click the Config button. On that page, under configuration, click Backups. Inside of there will be a list of everything that you have removed. Simply put a checkmark next to the item you want restored, and click, why, the restore button, duh. HijackThis will then move the item from its backup folder back to the way it found it.
Spyware is tricky, and mistakes get made. At least we have a way of recovering from most of them.
You're going to want to keep in mind that if you accidentally restore a spyware or malicious item (like a virus) that it will likely re-add its entries to everywhere in the registry again, so you'll need to clean once more with HJT. Just make sure that you're sure you want to restore the item so you don't suffer a re-infection.