Note: For the moment being UTAU seems to be working in Windows7 (64bit version), but as its execution has not been tested, please use it at your own risk. (Windows7 (64bit version) users have reported it to work in the UTAU exchange board -> http://utau2008.bbs.fc2.com/?act=reply&tid=3819714)
Warning: With Windows versions other than the ones mentioned above, there are errors in the creation of WAV file and it doesn't work properly. With Windows98 (SE) and Windows Me, there is a way of using which prevents the error, but we cannot recommend it because the UTAU application and the OS can crash suddenly, with the consequent risk that at worst the OS stops starting. If you really want to use it, carefully read Ameya/Ayame san's blog post http://utau2008.blog47.fc2.com/blog-entry-49.html#more, but use it at your own risk. Regarding this case, please understand that Ameya/Ayame san and Kenchan can't provide any support.
Note: Regarding the specifications of a suitable PC, the old notebook I was using before (made in 2002, WindowsXP HomeEdition SP2, AMD Duron 950MHz 256MB RAM) could, in 3 minutes, successfully read a (116KB) UST file (UTAU private project files), play it and export it to a WAV file, but when reading or drawing a particularly big project file, be careful that it may not execute very well on a PC whose memory capacity is small because this consumes a large amount of memory.
1-2. Installing UTAU (Described on WindowsXP)
As MimiRoboP san has created an easy to understand video on the operating instructions, please refer to it too.
1-3. Adding UTAU singers (voice banks) to the default singer
UTAU comes with a default voice bank, a female voice created with AquesTalk voice, but you can also add and use voice banks created and published by supporters, like e.g. my own voice banks or Kasane Teto.
Note: For a UTAU Voice introduction video (UTAU voices are referenced in the 2009 and 2010 editions of the NikoNiko Douga Encyclopedia on Voice Synthesis produced by Twinkle Zter san) -> UTAU voice bank introduction video
Note: Among the currently published voice banks, there are some in which sound setup is not done, or sound setup is done but may require re-adjustments. It is UTAU author's (Ameya/Ayame san) policy that the user performs the primary sound setup, and that publishing voice banks with primary sounds not prealably setup is admitted, thus do not request the setting of primary sounds to the voice contributor.
Warning: Never use VOCALOID's voice output as UTAU's voice bank. (Article 4 of the VOCALOID End User License Agreement applies: (Prohibitions)(3) "Using all or part of this product as a component of your or a third party software.")
2010/01/15 Addendum: Installing voice banks through the automatic installation feature available starting from Ver0.2.61.
Starting from UTAU version 0.2.61, a function has been added to automatically install voice banks, just by dragging and dropping adapted voice banks onto the main screen.
Note: The automatic installation is limited to valid voice banks, which are archive files built according to "install.txt" instructions, with a .ZIP or .UAR extension. A .ZIP file not conforming to "install.txt" cannot be installed by drag and drop and produces the following error, and must thus be installed manually -> Manually installing a voice bank the traditional way.
Note: You can update with the "Installing Over" 「上書きインストールする」 method of the latest version of the installer, but as the voice bank Settings of the primary sound are reinitialized, you must take care of making backups of the voice bank settings file "oto.ini" if necessary.
1-5. Registering UTAU shareware Ver0.2.61 and later
Until now, UTAU was published as free software, but starting from Ver0.2.61 it has been registered as a shareware with Vector and DL-MARKET. (The price is 3,400 yen including tax.)
However, you can still indefinitely use Ver0.2.61 and later as a free software without functional restrictions, as before, even if not registered. The only benefits of registering the shareware are small design changes in the main screen. UTAU's author would like to ask for a contribution if you have money to spare to support future developments of UTAU.
Note: Vector will only accept credit card as shareware payment method. With DL-MARKET, bank deposit is possible too.