ok dude yes it's possible to do it from a flash drive. I don't know how to do it and have never tried and doubt anyone here as tried it. With that said a search of google will give you what you need. It's going to be trial and error baby. if at first you don't succeed try try again.
One thing I'd say to try though is http://wintoflash.com/overview/en/
. If it works it should do the job for you in a very simple manner. If that doesn't work just search google. Here's one FAQ to start you out: http://www.ocia.net/articles/xpflashdrive/page1.shtml
. but dude really try the winto flash thing first. Also make sure whatever copy of xp you use works in the first place before you decide anything has failed. Just burn it off on a disc and see if the installer boots up on your desktop. I would recommend using microxp. after you get it on the usb of course follow the tutorial in this forum. If you need to know where to find a copy of microxp all I can tell you is to walk to the BAY with the excessive amount of parrots.
Ok what ever goodluck