As stated above, because Win 7 does not appear under the OS ?

I have it running on 32bit Windows 7. I don't know about 64 bit.

However, caveat emptor - Blood 2 is fairly unstable under any system from my experience.

I have 64-bit, so it should work if you have it.

While people in this forum have had many issues and even some of my friends, I've never had any. But if you do, I'm sure a quick skim through the forum will find you your answer.