The following description will show you how to install FHEM on Windows on a separate
USB-Drive, without any Windows-registry modifications.
You can use  the internal HD for installation too, and you can register fhem as
a service, see below.

Install FHEM:
  Download the latest fhem-X.Y.tar.gz package from http://fhem.de#Download
  (currently it is fhem-5.4.tar.gz), and unpack it into a directory where you
  have at least 350MB+ free space. I will use F:\tmp for this purpose, after
  unpacking the .tar.gz file we will have a new directory F:\tmp\fhem-5.4


Install perl:
  Download a Strawberry perl Portable-Edition (this version won't
  generate Registry-Entries, and can be installed on USB disks) from
  http://strawberryperl.com/releases.html, and extract it into the previously
  created directory, in our case F:\tmp\fhem-5.4
  My file was called strawberry-perl-5.16.2.1-32bit-portable.zip


Start FHEM:
  Open a command line window (cmd), change to the previous directory, and start
  FHEM:
    C:> F:
    F:> cd \tmp\fhem-5.4
    F:\tmp\fhem-5.4> mkdir log
    F:\tmp\fhem-5.4> perl\bin\perl fhem.pl fhem.cfg
  Note:
    There is no visible output and the command won't terminate.
    You probably have to confirm a Firewall exception (Do not block), as FHEM
    opens a Web-Server Port.


Connect to the FHEM Web frontend (FHEMWEB):
  Start your browser (Firefox, Chrome or Safari are preferred) and open
    http://localhost:8083/fhem
  You'll see a smiling-house icon on a light-yellow background.


Update FHEM to get access to the latest development version (recommended but
not mandatory):
  In the browser window, on the FHEMWEB page, in the command line (upper text
  input field, right of the smiling house icon), type in the following 4
  commands one-by-one, confirming each one with Enter:
     notice confirm update-20130127-001
     attr global backup_before_update 0
     save
     update
  Note:
  - the last command will take a while (around a minute), and after finished
    it will ask you to "shutdown restart". Ignore this, and just type
    shutdown in the FHEMWEB command line.
  - Switch to the command line window, and restart FHEM with the last command,
    i.e. arrow up and RETURN or type in 
       perl\bin\perl fhem.pl fhem.cfg
    again.


Install FHEM as a service (better to install perl on the internal hard-disk for
this scenario):
  Terminate fhem by typing shutdown again in the FHEMWEB command line.
  Install the missing Win32::Daemon perl module by typing in the command window:
    F:\tmp\fhem-5.4> PATH=F:\tmp\fhem-5.4\c\bin;F:\tmp\fhem-5.4\perl\bin;%PATH%
    F:\tmp\fhem-5.4> perl\bin\cpan -i Win32::Daemon
  Install FHEM as a service
    F:\tmp\fhem-5.4> perl\bin\perl fhem.pl fhem.cfg -i