How to boot a Acer laptop that has a operating system installed on an inactive p

I have 3 partitions on a ACER laptop. C: is Primary formated but has no operating system installed after formatting. D: is Primary and has Vista Business installed; E: is unformated logical partition.
Initially D: with Vista was active, and C: without an operating system was inactive.
Trying to install Windows Server 2003 (which was not allowed because the installation configuration feature analyzed Vista and concluded it was a superior operating system) I made the mistake to activate C: and made the partition with the operating system inactive, so I can't boot anymore.
If I press F2 during the start I get a "Enter Password" screen that I never used before, so I don't (never knew) waht is the password to enter setup.
Could somebody help me, as I really need to install an operating system to do my .asp scripts for my INT class ASAP ?