As an upgrade you can install if existing Winxp is with Sp1, you can
upgrade it to windows xp with xp2 , this will not wipe your data. all
the data including your profiles willbe intact. to do this your
existing O/s should be working ( i mean it should boot properly and you
should be able to logon)
As a dual boot you will be running 2 instances of o/s, not recommended,
if you are really facing some kind of problem with existing O/s you can
install as dual boot.
to do boot from the cd and select the same partition with different
folder or choose different partion. do not format the partion where
data exists (system partion and data partion) format only free space.
after installing as dual boot all applications, service packs and
updates has to be reinstalled.
Repair is onther option which is recommended in case of O/s crash not
Hard disk Crash.
repair will also keep your data intact. only thing you should reinstall
Service packs, Updates. applications will remain and work as previous
Thanks
Bhaskar.G
MCSE