How to get rid of the school's domain off my laptop?

My school have put their domain on my laptop ( at least I think it called a domain) so when I load it up, everything I have on the school computers desktop appears ect... Now that all very well, but I'm leaving the school ( Aside from 2 exams, I basically have left) and now want my laptop back, in the sense of freedom as a user. Currently I can't install or download software without an administrator account, so cannot watch dvd's as the school didnt care to add PowerDVD. And I can't make myself administrator with an administrator account. And I can't change the domain without an administrator account. The school will probably be unwilling to take it all off, as they are adamant that they are keeping me for sixth form, and I'd prefer to not have to pay PC world to wipe it. I should be okay reinstalling windows, and everything else ( I think at least, I'm not sure whether the laptop came with a windows 7 disk or not) but I need to either get rid of all of the schools changes to it, or start again. Any advice or instructions on what to do is greatly appreciated, even if inconplete.