Kerly2-Bill wrote:

Is it possible to run more than one copy of the S@H on one computer?  I'm running Windows XP Pro, more than one hard drive, 512 Mb of memory,

Anything is possible, but unless your processor uses hyperthreading or something similar you will not be performing any additional work for the project.

One copy of S@H utilizes all of your available processing power. Running two copies (barring the hyperthreading mentioned above) will use an equal fraction of your available processing power for each copy.

If you are really hyped about this distributed computing thing why not explore some other BOINC projects that are available and run several projects at once?
http://boinc.berkeley.edu/