Might be because the software is tremendously out of date. That being said, I've successfully installed Photoshop 6.0 on a Win 8.1 system.
Are you hoping to only install Photoshop 5.0 so that the CS upgrade will see it?
I have bad news and good news for you: Even if you succeeded in the first part of this plan, you would never be able to activate Photoshop CS - the activation servers for Photoshop CS and CS2 are long gone. Offline. Kaput. Finito.
That was the bad news.
Now for the good news, which will eliminate the need for you to install 5.0 at all (unless you really want it, in which case we may be able to work out a way). Go through the following, thinking "CS" where it says "CS2".
The so-so news is that Adobe seems to be having a few problems with their servers. But be patient and try again if it doesn't load, the page really is there.