OK, full story. I was meddling a bit with the security settings for my C: partition (where my Windows 7 resides) by right clicking the drive and selecting properties > security. The reason for doing so was because I thought someone had meddled with it first, so I was trying to get it back to the default settings by using the same settings as my other two partitions. It turns out the settings for Authenticated Users for C was different from both the other partitions, so I changed them to reflect the settings from the other two. Later on I found out nobody had tampered with the settings before me (but why then did the security settings for different partition differ? Can someone explain?), so I attempted to revert the settings by using system restore.
After the system restore, my main Admin account was available but the other accounts (there were 3 others) were missing. I tried system restoring back to a few days, didn't solve it. I ended up using system restore a few time (back and forth between a few restore points), before it suddenly turned up again. Why would this happen?
Also, the system restore did not change back my security settings for Authenticated Users. Do you think this will affect anything?
This is my current security settings: (Note the "special permissions" under "write" is unticked)