First, you need to set up your computer so you can boot to either Vista or,
an earlier OS, such as Windows XP.

Usually, you should install the older OS first.
Each OS must be in a separate partition.

Having multiple OS does not affect performance.
When you  are booted to one OS, the other OS is not running.

If  Excel 95 can run in Windows XP, then just boot to XP and install Office
95.
Has no effect on what goes on when you are booted into Vista.

Check out this site.

http://www.computerhope.com/dwin.htm

If your system has the resources for it, you might find this an interesting
alternative to partitioning your drives...

http://www.microsoft.com/windows/downloads/virtualpc/default.mspx

I have it set up on my 64-bit Vista laptop (don't ask<g>) and what it does
is let me run Windows XP inside a window on my Vista computer. That means I
can switch between Windows Vista programs and Windows XP programs as easily
a I can switch between any two running applications. It really is quite
neat! And best of all, the program is free (of course, you need a legitimate
copy of Windows XP to install into it, but I assume from your question that
you already have it).

Rick

One more -

Have you actually tried to install your Excel 95 on Vista. If not why not
give it a go, ignore all warnings. FWIW, although I haven't installed XL'95
I have installed one or two other very old both W3.1 & W95 app's in Vista,
no problems despite the warnings.

One thing though, before attempting to do that, fully uninstall any other
versions of Excel as they should always be installed in order of oldest
first. Also don't install into a default "Office" folder but say into
"Office95"

Regards,
Peter T

Vitural PC is the best solution.  you can have as many operating system
running as you want as long as you have enough space on your hard drive.