I replaced the hard drive with one that has OS on it, but I don't remember it's user password.
Is there any way to format my current win7 without a OS installation disk and without access to my current win7?
No, you'll need a new OS or a disk where you installed your previous OS from (if it was preinstalled contact ur old store/microsoft, maybe they can give you a new installation disk with your old OS code)
Also are you positive it doesn't have a OS? Isn't the hard-drive just ded? - I mean I know it doesn't have an OS, but is that the only problem?
If you don't have an operating system on the hard drive you are using to run your computer on -> it will not boot lol
If the hard drive is dead then it won't boot either, but then your OS is probably corrupted and you need a new drive (with new OS or your old OS installing on it) to boot again.
IF it's not dead (if the OS is just deleted from it but the drive itself works fine) get a new installation disk, and run it so you install an OS on it, should work fine.
Edit: It doesn't matter anymore, I could not fix it in time. Probably doesn't have any other solution than either
Reinstall OS
RAM chip or chips dead
Could also be ram chips broken, in both cases your pc won't boot. I would definitely first try if you can get a new disk from your store/microsoft with your existing windows code (you can find it on a silver sticker on your pc) and see if you can reinstall. If not it means your hardware (harddrive/ram) is broken.