Virtual Piano is a program that enables you to play piano or any other available instruments with your computer keyboard. Well, it's not the most convenient thing, but it's better that nothing. Alternatively, check out MIDI controllers such as this.

zsxdcvgbhnjm	C C# D ... through B, first octave
q2w3er5t6y7u	C C# D ... through B, second octave
i9o0p[=] BACKSPACE BACKSLASH	C C# D ... through B, third octave
spacebar	pedal
left/right arrow	change instrument (out of 128 instruments)
page up/down	change octave offset
enter	reset (stop all notes)

Before downloading, you will need to have Java runtime environment. The minimum release version of the JRE is unknown, but if you have JRE 1.5 probably you're fine. Java JRE can be downloaded at: http://www.java.com/en/download/index.jsp

You also need to have Java soundbank, that can be downloaded from any of these links:
Minimal (0.35 MB) / Midsize (1.09 MB) / Deluxe (4.92 MB)
Unzip the file, and put the file in your "C:\Program Files\Java\jre1.6.0\lib\audio" folder or a similar folder depending on your Java JRE version and your operating system.

Then, download Virtual Piano: VirtualPiano.jar

Note that different computers may produce different sounds.

There is no license for this program. If you're happy using this program, I'm happy too. And if you're not, feel free to fix or modify the program. Feel free to use the program for other purposes.