I had a similar issue about a year ago when a client asked me to do some updates to her sCommerce store. I had used eCart 3 to build it with DW8 about 4 years ago now.
I now run DW CS4 and have eCart 5 installed.
So I kept all my current installations but also installed DW8. Both versions run independently from each other. There are also two different extension manages that are used so no problem with having two versions of extensions installed either. Worked like a dream.
I seem to remember having to get a patch so that DW worked on Vista at the time but that worked OK as well. I can't remember where I got the patch from, I can only just remember that I needed one. There may be a thread on these forums about that. I really can't remember.
Hope that helps.
And eCart 5 is only PHP. It will not work with ASP.