Recommend me a bootable anti-malware USB thingie

[DeeBye]

I have a computer that is horribly infected. I can't run or install anything on it when windows boots up. Normally I would just wipe it and re-install windows, but it is a pre-built and I don't have the windows key. I'll need to run something like Productkey to get it, but I can't install anything.

The two major infections are Remote Desktop Manager and Windows Security Alert. No, the computer is not connected to my network.