I don't think a re-install is required. Although RedHat does not support NTFS filesystems out of the box, there are workarounds for this. Mandrake, OTOH, suports NTFS filesystems, so you can read from (but generally not write to) your NTFS/WinXP filesystems from Linux.
Now, the dual boot issue: you could certainly set things up so that XP is your default boot choice; if you do nothing, XP boots. If you select Linux, Linux boots. Without going into details, I believe you can use either XP to set this up or Linux (i.e., you can choose a boot loader from either O/S and still dual boot). You pretty much have to use a dual boot system or a Linux distribution that runs entirely from CD (such as Knoppix) if you want access to more than one O/S. In general, setting up a dual boot system is not difficult, although it may be confusing and a little intimidating if you haven't done it before.
Not sure what your concerns are with dual booting, but anyone who is willing to tackle Linux can certainly handle dual booting (

).
Hope this helps.