Nope, it's not. Windows Vista Home Premium x32:

Relevant section of php.ini:

; Windows Extensions
; Note that ODBC support is built in, so no dll is needed for it.
; Note that many DLL files are located in the extensions/ (PHP 4) ext/ (PHP 5)
; extension folders as well as the separate PECL DLL download (PHP 5).
; Be sure to appropriately set the extension_dir directive.

;extension=php_apc.dll
;extension=php_apd.dll
;extension=php_bcompiler.dll
;extension=php_bitset.dll
;extension=php_blenc.dll
;extension=php_bz2.dll
;extension=php_bz2_filter.dll
;extension=php_classkit.dll
;extension=php_cpdf.dll
;extension=php_crack.dll
extension=php_curl.dll
;extension=php_cvsclient.dll
;extension=php_db.dll
;extension=php_dba.dll
;extension=php_dbase.dll
;extension=php_dbx.dll

Proof it's not working:

c:\users\miff>curl http://localhost/xampp/phpinfo.php | grep curl
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 62592    0 62592    0     0  1971k      0 --:--:-- --:--:-- --:--:-- 3319k
<tr><td class="e">HTTP_USER_AGENT </td><td class="v">curl/7.16.3 (i686-pc-cygwin) libcurl/7.16.3 OpenSSL/0.9.8k zlib/1.2.3 libssh2/0.15-CVS </td></tr>
<tr><td class="e">User-Agent </td><td class="v">curl/7.16.3 (i686-pc-cygwin) libcurl/7.16.3 OpenSSL/0.9.8k zlib/1.2.3 libssh2/0.15-CVS </td></tr>
<tr><td class="e">_SERVER["HTTP_USER_AGENT"]</td><td class="v">curl/7.16.3 (i686-pc-cygwin) libcurl/7.16.3 OpenSSL/0.9.8k zlib/1.2.3 libssh2/0.15-CVS</td></tr>

c:\users\miff>