My boyfriend took over my computer and put himself as administrator, how can I get past his password on my secondary account?

Bad Bad BF.
You didn't state XP or Vista.

Do reading here for Vista

The only issue here, sorry but what if this is his computer and you want us to tell you how to bascly hack into it and get past his passwords.

Thus the issue with helping someone and not knowing the computer.

I would suggest telling boyfriend to give you the codes or don't call again
Perhaps then don't see him afte ryou get the codes anyway.

Lolzz someone is clever to ask how to hack into computer.
Well , I could suggest you simple command NET USER

Its you luck if this works for you (you just need to modify it a bit)