How do I reset a local admin password without another admin password?

My computer is a computer owned by myself and myself alone, though it used to be shared between myself and my sister when we were young, managed by our dad. I have 2 accounts on it, a personal user account that I primarily use, and a local admin account, previously controlled by my parents, vestigial now that I'm the sole owner of the computer. In hindsight, I should have simply used the admin account to make my main account an administrator and then deleted the admin account, but I've gone and forgotten the password. Since they'd set it up to have the password expire every month or so, I constantly had to change it, and I've lost track of what I set it to.

At one point, I tried a certain password (which I'm 80% sure was the right one) and it hung on "Welcome..." for a bit, as if it were correct and it were about to let me through, before just telling me the password was incorrect. Every time I've tried that password, it's done the same thing, and no other password has had any such result.

Unfortunately, all the methods I've found online for getting into a locked admin account without its password require another admin account (RE command prompt, safe mode with command prompt, etc). I don't have my old windows install disc, nor do I have a USB drive handy to do whatever process is involved in password recovery (nor am I particularly fond of the idea of all that), though I could get one. It's unencrypted since it's a personal computer and all, but I've failed to find anything that works.

Not having access has turned out to be quite a pain, as I can't really install anything. I'm willing to try pretty much anything at this point.