Thank you all for the replies. My first post could confuse some buddies because I haven't described the details of my trading.
I built this desktop only for trading due to safe and convenient reasons. I don't need to play games or other programs, except trading software, on this machine. I had two choices for the operating system. : XP 32-bit or Win 7 64-bit. The former is the recommended one by trading companies and the latter is more powerful and faster. My daily use computer is a laptop with an OEM edition Vista.
Depending on your replies, I think my best choice is XP right now. As Scataphagos, redneck and other buddies said, the safety and functions are the merits of XP. A newbie in stock market doesn't need many advanced software for trading as of today. Win 7 seems too powerful for me. Petsamo, nettecturs and other buddies also mentioned that Win 7 will become the standard of trading software in near future. So I may upgrade XP to Win 7 after I gain much experience in stock market. The major trading companies will strongly support Win 7 then.
Thank you all again for replies
Jerry