Serial to USB converter
A1: By far the quickest and easiest method, and possibly the equal best, is to buy an existing USB to serial converter. I've seen these advertised on ebay for under $5 and those were probably not the cheapest. Here are ebay's listings of USB to RS232 converters - well worth looking at to see the range available (and prices). 

Wow! - here is one for $US3 including shipping to the US from Hong Kong. 

OK - checks - you're in Mumbai, India - checks - $US1.99 to India, free shipping!. You won't build one for that little - probably by a factor of 10 to 20!

Here are a few dozen photos and links to serial to USB converers - hover over the pictures for more information (on the page linked to above - not on the picture below) - click to go to the related site. A few examples - lots of choice !!! - 



You can build your own using any of several available custom ICs created for this purpose - but doing that will cost you more than buying a prebuilt one.

A market leader but not the only source of ICs is FTDI corp who make these products.

A2. The most common units (usually a "cable" that plugs into a USB port) appear to a PC as a standard serial "comms port". They work across a range of baud rates. May be manufacturer dependant. Will be settable in software. 

A3: As above - appears as a standard serial port. 

A4: Some manufacturers peripherals do strange things and may not work for all converters. For example the PIC based PICAxe system uses a programming protocol that sends all data to and fro using RS232 "break signals". ie they do not use the data lines for information transfer. Some converters do not support the break signal.   

You will need to know if your equipment is grossly non standard. If it is, some converters may work with it.

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