I am trying to make an Alarm system which involves an Infra-red circuit which is connected to the serial port on the PC and generates a signal when motion is detected. The signal is to be read by a Java program and an e-mail sent to the client. Does anyone know how the circuit is to be made?

Maybe use an Arduino? They have IR modules available.

are Arduino's readily available in the market?...what kind of signal is to be generated at the COM port for a PC to detect it?...is it a voltage difference or a logic value?

Yes, they are very common. Small prototyping boards. Under £50.

http://www.coolcomponents.co.uk/catalog/index.php?cPath=50

I have made a system where my Arduino was reading the values of a light and temperature sensor. It was processing these and adjusting the power to a light and heater appropriately, all the while it was sending logging data over the serial port to a PC (binary ASCII data, you can view this using say, hypertrminal, or read it in a few lines in Java or C#), on the PC it was made into a graph and shown on a web page.

I wrote the code in under a week (more like a day).

hey thanks a lot....actually i ended up using a basic IR circuit with a TSOP sensor....i guess it should do...i'll order an Arduino if i can delay the project submission date a little bit...i'll read the data from the hyperterminal i guess...thanks again !