It's probably Microsoft themselves that leaks these builds, and the beta keys still works with them, so no big deal imo. If anything, they get large scale testing out of it, which in turn leads to a better product.
Would be quite expensive buying a new hard drive every time you got some malware on it, considering it's quite easy to clean it if you know what you're doing.
Your memory card is probably infected, so when you run stuff from it, it infects Windows. Try to run Windows in safe mode with networking, then download malwarebytes and superantispyware and run a scan.
DOSBox is an emulator with DOS, which you can use to run old DOS based programs in. Should be pretty straight forward to get it running if you read up on it.