It would seem that the Haskell Platform for windows only ships a 32 bit binary. That is rather unfortunate:
$ ghci +RTS --info
WARNING: GHCi invoked via 'ghci.exe' in *nix-like shells (cygwin-bash, in partic
ular)
doesn't handle Ctrl-C well; use the 'ghcii.sh' shell wrapper instead
[("GHC RTS", "YES")
,("GHC version", "7.0.2")
,("RTS way", "rts_thr")
,("Build platform", "i386-unknown-mingw32")
,("Build architecture", "i386")
,("Build OS", "mingw32")
,("Build vendor", "unknown")
,("Host platform", "i386-unknown-mingw32")
,("Host architecture", "i386")
,("Host OS", "mingw32")
,("Host vendor", "unknown")
,("Target platform", "i386-unknown-mingw32")
,("Target architecture", "i386")
,("Target OS", "mingw32")
,("Target vendor", "unknown")
,("Word size", "32")
,("Compiler unregisterised", "NO")
,("Tables next to code", "YES")
]
With a 64bit build, perhaps I could experiment more. I looked at the Haskell Platform page and I don't see a 64bit version. It would seem the binary from GHC HQ has the same limitation since there is only one installer and it claims to work on windows 2000: