|On second thought, Flash Player 11 requires SSE2 instruction which your 600MHz Pentium III/Celeron doesn't support. You need at least a Pentium 4. Likely why it won't install.|
I do remember the system requirements going from Flash 10 to Flash 11 were quite a bit steep, I think 10 required a 700MHz while 11 requires >2.2GHz.
I'm afraid you're out of luck and will have to stick with Flash 10,1,102,64 (10.1.102.64) for Internet Explorer, Flash Player 10,1,102,64 (10.1.102.64) for Firefox/Opera, or upgrade to a SSE2 processor.