Wow, this is confusing to me!
The last MS-DOS was version 8, produced in year 2000. Version 5 was made between 1991 and 1993. Maybe the question was about NT
version 5 and 6 ? Of course NT is build upon MS-DOS, but that doesn't make it MS-DOS, it is still NT. NT 5 was used for XP, and NT 6 for Vista. I will assume (as Josh obviously did) that the question is about NT ?? However, my worry is that if you fail to understand the difference between MS-DOS 6 and NT 6, then you're bound to make some crucial mistake.
So my advice would be not to go alone
, if you insist on upgrading (upgrading from XP to Vista, that is; it is impossible to "upgrade" to MS-DOS, because Microsoft stopped using MS-DOS ten (10) years ago!).
or maybe you really want an original MS-DOS 6.22
from June 1994
-it was the last MS-DOS to be launched without Windows!