Don't have administrator rights to upgrade software

Ok guys... thanks in advance to anyone that can help me with this... I don't even know if I am posting this on the right section... but here I go...
I just got this awesome MacBook, black and works great... I am even writing all these words with the one I am having the issue with... the thing is I CAN NOT UPGRADE ANYTHING because I am not the administrator.
The INTERNET works great, all the features are amazing... and to be honest I bought it in a Pawn Shop... who knows where is this computer coming from... .... but I paid for it and I am happy with it...
Don't know the password and now I am kinda like... hmmm.
Can I use another MacBook Software Install and Store DVD?
Any advice will be nice....
Here a few data of the computer....

Procesor: 2.16 GHz Intel Core 2 Duo
Memory: 1 GB 667 MHz DDR2 SDRAM
Startup Disk: Macintosh HD

Yes, It was a good deal at the store and I couldn't help it.
Thanks for the advice and not judging...

NOTE: Not a hardware issue. Moved to Software forum.

G'day and welcome to the forums.

You can try the other System Restore discs but usually they work only with model they came with but finger crossed. Whether it works or not, suggest purchasing a full retail install DVD version of Leopard as it is the best repair utility available to you.