With Windows, No (due to hardware changes, product activation blah, blah, blah); with Linux, Yes, most (but not all) distributions support booting from USB.
Check out puppy Linux. Another thing you can do is run XP on a virtual machine from an external drive, that way whatever you do on there stays on there.
------------------------------Playing X-Men Origins: Wolverine Athlon 64 X2 5000+ @3.24 Brisbane | GIGABYTE GA-MA790X-DS4 | 4GB Mushkin DDR2 1066 | Plextor 760A| 2x 3850 512M CF| WD 1TB Black| Fortron Blue Storm II 500W | APEVIA X-Dreamer Black | Win XP Pro & Vista Buisness 32bit
Reply to megamanx00