HELP: how to set a partition as active or system

i want to set my third partition as system. because i have my windows 7 there clean installed not like in my system partition. i want to do this to make sure that any other errors and bugs from the previous OS in my system partition wont be transfered to my windows 7 there.

i tried to set it active through EASEUS and CMD but both no luck. my machine wont boot. i managed to get my machine online by setting my system partition active again with the help of SIW2.

can anyone teach me? or is it even possible? thanks!:)

i actually have triple boot. what i want to do is set my other primary partition as "system or active" and then delete my previous (which as of now is my current) system partition.

i stumbled on these MS support sites but i can't fully understand them because i'm not much of a computer wizard.

http://support.microsoft.com/kb/315261
http://support.microsoft.com/kb/228004

i know there's an option to format my system partition before installing windows 7 to do a vary clean install. but i install from the setup disc in my desktop so there is no format options when installing.

geez, i always go all these trouble becasue i dont have a empty DVD. hahaha