Let's be clear. When I say "active", I don't mean its use as a general English synonym for "in use".
I'm referring to switching the "active" flag in the MBR to point to a particular partition. That's the flag which the MBR uses to locate the boot sector and controls how the boot progresses. Only one partition on each HDD can be active at any moment.
It's also examined by the MS setup during installation and controls where setup places the boot files, and whether or not it automatically creates a dual-boot..
If you install W7 with XP "active", you'll get an automatic dual-boot and boot files for both XP and W7 will be in the XP partition.
If you install W7 into an empty "active" partition on the same HDD as XP, you'll get a W7-only boot with W7's boot files in its own partition. XP will still contain the XP boot files, but you will only be able to boot XP after adding an XP entry to the W7 BCD yourself, or by switching the "active" flag back to point at XP.
Whichever you choose, you cannot change horses in mid stream and change the "active" flag during the install.
You also didn't mention additional HDDs in your OP.
They will complicate the possible permutations of what happened to your W7 boot files.
W7's annoying default behaviour during installation is to attempt to place the boot files in a small dedicated partition which it calls "System Reserved". It will do this if you allow it to use empty space on the disk. It divides the space in two (100Mb for the boot files and the rest for the OS).
You can prevent it from taking this action by formatting the space as a single partition before starting setup, but if the space is not set "active", it will still separate the boot files by searching for an "active" partition elsewhere, primarily on the same HDD, but if that's not possible (no active partition or insufficient space) it will place the boot files on the first active partition it can find on any other HDD.
It seems likely that that's what's happened to you.
Set your folder options to be able to
view super-hidden files and look on the other HDDs for bootmgr and \boot.
If you find them, set that HDD to be top of the HDD priority list in your BIOS boot sequence.