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The game installation consists of two things. First, the installer copies the files to your hard drive. Second it creates some crucial 'keys' or data sets in the Windows Registry. If you can't use the installer if you're running Windows 7 or Vista you can still setup the Registry manually. Good luck.
After downloading the reg(istry) files, edit them first (Right-click, “Edit”) and correct the paths. All occurrences of
"Executable"="c:\\games\\XVT\\Z_xvt__.exe" "Install Path"="c:\\games\\XVT"
have to match your installation path. Note that backslashes have to be escaped ('\' → '\\') in the reg file syntax.
Download xvt_bop32.reg and edit it. Make sure the paths to your game are correct. The CD-Rom drive value, I think, isn't all too important, but it doesn't hurt if it's correct either. Then import it into your registry. It will add XvT and BoP paths, so keep that in mind when you install BoP.
Same as above with 32-bit, but the registry path is slightly different (Wow6432Node). Download xvt_bop64.reg.
Download xwa.reg and edit it. It's similar to XvT.