There have been a couple threads on this recently.
Would Windows XP boot faster if I booted it from a flash card using an Compact Flash to IDE Adapter? All mp3's and gps maps could be stored on a regular HD. I think this would create fastest boot times, correct me if I am wrong, or if there is some flaw I am not thinking about
A search will show you that it's indeed a bit slower using a CF/IDE Adapter. The effort put in to make XP not write to the CF disk is also a bit prohibitive. Your best bet is a laptop hard drive.