The different versions of MSDART work with different versions of Windows. So, there's one for XP, one for Vista...and so on. And you can only use the one that's compatible with the OS that's installed. So - tbh, the error message you're getting makes me wonder if if it actually is Win 7 on this machine. OK - it has a sticker to say so, but you haven't actually seen it boot up to prove it...you could drop a Linux or Win PE CD in to see if you can see the file system or better still, get to the registry hive. If you know what you're looking for, you should be able to puzzle out exactly which OS is installed.
If it genuinely is Win 7 but is so shagged that msdart etc can't even recognise it, then an N & P is, as others have hinted, probably going to be your only way.