Assuming that your copy of Windows 7 is a 64-bit version, why not try this link over at ATI.com? Otherwise, you could check for the 32-bit version.

Also, what you described is not necessarily that you lack a 3D accelerator to properly run Blender, it's that you actually lack the OpenGL driver that Blender is heavily dependent on to run, hence the message. This usually happens if you've recently installed Windows (generally any copy from 98 to recent does this) and it installs a baseline driver that works on your video card, but does not provide any additional functions. The only thing you can currently do, especially if the problem keeps happening, is try to use a driver cleaner to remove any and all drivers for your video card, then reinstall the drivers from either the cards manufacture or ATI's website.

Hope this helps some.