There may well be issues with running old, 16-bit, MS-DOS apps on modern, 32-bit, multitasking/multithreaded Windoze platforms - especially if they try to do cunning tricks with extended/expanded memory, direct-hardware dodgy tricks, or other "DOS extender" techniques.

BTW: Keil's command-line tools are proper 32-bit console-mode applications; perhaps it's time for an upgrade?

Or you can download Raisonance's 4 KB evaluation software and get a 4KB assembler that is a 32 bit application and loads it's code at 0000H. www.raisonance.com or www.amrai.com.