Photoshop 5.5 & 6.0 wont open or ins...

I have been using photoshop 5.5 for years. All of a sudden it won't open or allow me to re-install (the setup file won't run). It gets as far as the blank (black) photoshop window but no further (no drop down menus etc). The install won't even start.
I tried getting a copy of version 6 from a friend but it won't install either. I have established that it is not my VET firewall causing this so there must be some other conflict going on.
My computer is operating with windows 2000 service pack 4. Its an Intel Core 2 CPU 6600 @2.4 GHz 2,100,00 KB RAM

Any help would be appreciated.
After installation,  when I open  Photoshop 6  everything is there except the the menu bar. I use Mac OS10.4 with classic facility. Have tried reinstalling and can get full screen once but if I quit and try to open again the bar is again missing.  Same result whether I try direct from installed files or from alias in classic launcher or alias in OS10 dock. When used in OS9.2 program worked perfectly