8051 Compiler

I don't work in Work in windows any more; but, I used to use a
multi-processor, table based assembler called TASM.  It worked well for my
purposes at the time.  Cannot be sure that it works under Vista.  The best
that I can tell is that it is shareware and the home page seems to be:

http://home.comcast.net/~tasm/index.htm

It may be worth your while to try.  If it doesn't work directly under
vista, you might still use it inside of DOSbox on Windows:

http://www.dosbox.com /

Alternatively, you could install a small partition with DOS 6.22 or
FreeDOS.

snipped-for-privacy@ilthio.net says...

DOS Box is probably the best way to go.
I use it under Vista 64 Ultimate to run a bunch of old DOS tools.
Just try to stay away from long filenames, all that ~1, ~2 crap can get
old.

You might try this, running under cygwin on WinDoze.  I use cygwin
regularly under XP and it is rock solid.  I've not used this compiler,
but it's been around a long time now:

  http://sdcc.sourceforge.net/